Been such a long time since I thought about this topic but felt like I can add a little bit more after having had interacted with the French across the years.
Assuming you must study in France, if you are non-French and want to work either in France or outside of France, go for INSEAD. If you had access to the Grandes Ecole process, then HEC Paris > ABSOLUTELY everyone. This is just my opinion as of now and would likely change within 5 years given how fast HEC Paris's MBA is improving.
As a matter of context, one needs to understand how the French tertiary system works. This is a bit of a history lesson, but you will thank me because you will come to understand why the French have certain preferences. The below will also help you understand the mindset of the French execs you will deal with on a regular basis in the future and why they recruit the way that they do (and maybe even why they treat YOU the way that they do).
In France, there are two parallel tertiary educational routes. There is the University route and then there is the Grandes Ecole route. The primary difference between the two, in my opinion, is that Universities are NOT allowed to turn anyone away (essentially, open access in the sense that if you have a high school diploma you get a seat, but you are eliminated during your final exams at the end of first year) whereas Grandes Ecoles can eliminate people via entrance exams. To put it into perspective, "Universite" is equivalent to community college in France. The Sorbonne you hear so much about? Trash. It doesn't even technically exist anymore (was dismantled in 1970s and now has 13 successor Universities). The Sorbonne has no been considered elite for over 200 years now (the French have known this for a long time, foreigners are still struggling with this because our media is still catching up with reality). Also, keep in mind that tertiary education in France, both University AND Grandes Ecole is state funded. Universities charge tuition along the lines of a couple hundred dollars total a semester or so. Grandes Ecoles? Some of them actually give you a salary for studying there! You can imagine how this affects the perception, by the public, of the prestige of Grandes Ecoles over Universities. Poverty wouldn't prevent you from giving it a shot, though I cannot deny that those that are better heeled would likely be better prepared.
In total, only 5% of all tertiary seats in France are Grandes Ecole seats. To take the examination for admissions into a Grandes Ecole you must first go through 2 years of prep classes (only the best students go through this process). If you fail to get into a Grandes Ecole after having done the 2 year prep classes you can transfer the credit to a University so you don't end up wasting those 2 years in any event. Basically, the rewards are high and the risks are low. Those lucky enough to be admitted to Grandes are considered the elite of France. Even among Grandes Ecoles, there is a hierarchy, and the hierarchy is often very clear: for Engineering, Ecole Polytechnique is at the top; for science/research, Ecole Normale Superieure is at the top; for politics, Ecole Normale d'Administration (Sciences Po is the typical prep school for the entrance exam for this one); and for business, it is HEC. Getting into ANY of these schools will set you up for life. Out of these, Ecole Polytechnique, Ecole Normale d'Administration, and HEC probably produce the most high level business executives in France. The reasons are obvious. Engineers (Ecole Polytechnique graduates) often end up as execs at more technical companies (aerospace, mining, automobiles etc...); top politicians (ENA graduates) are often given cozy positions after the gov't stint; and HEC produces the guys that run the finance companies and such whereas Ecole Normale Superieure, though also having the best students, typically produce guys that end up as researchers with no sense for business and no Silicon Valley equivalent in France to take advantage of their ideas (in my opinion one of France's main weaknesses).
For our purposes, lets put things into perspective. In 2014, 5194 students applied for HEC after having done 2 years of intensive preparation for the exam. Out of these students, 702 students were offered interviews. Essentially, only 13% of students are offered interviews. At the end of it all, 380 students are offered admissions. This is an admission rate of 7.3%. Of the 380 students offered a seat, 377 accepted the offer for a yield of 99.2%. The admission rate is competitive with any Ivy League school and the yield FAR surpasses any of them (to give you perspective, Harvard MBA has a yield of 89%, meaning 11% of those made an offer by Harvard rejects the offer, so 11% of admitted students consider doing something else or going somewhere else is better whereas only 1% of HEC Grandes Ecole admits think so!). Think that's impressive? The numbers are even more intense for Ecole Polytechnique and Ecole Normale d'Administration! But lets skip those since they are Engineering and Political schools and not so much business (though they probably produce the same raw number of top business execs in France despite having smaller class sizes in some cases... and as a side-note: if you want to know who has the prestige in France, study up on the position of Engineers in France. They hold the key to French elitism in many ways... in no other country do Engineers have so much sway and it actually confused me at first but in France even Doctors and Lawyers pale in comparison to the Engineer).
The culture of French business is this: France prides itself in being a "meritocracy." Basically, they believe that anyone who has succeeded and gained a seat into a Grandes Ecole, and particularly so if you gained a seat at one of the best of them, is someone special and deserves special treatment. You gain a special position within French society as someone who has, through merit, proven yourself. This system has been criticized because the French system emphasizes scholastic merit and not professional merit. Simply because you are a Grandes Ecole graduate, everything you say has more weight. Your mediocre ideas are now given far more weight than the good ideas of someone who did not graduate from the same school as you. Basically, you have gained a massive career advantage that will last decades, if not your entire career. You basically have to shoot yourself in the foot for a decade straight before people start questioning you.
Now that you KNOW the above, imagine yourself as a French recruiter for a top notch French company that has graduated from a Grandes Ecole. Do you think that they would treat you, a foreign graduate of HEC who has NOT gone through the same rigorous testing that they did in their youth, the same way that they would treat a French national who went through the same selection process they did? They won't. They won't treat your HEC MBA the same as the HEC Grandes Ecole education they themselves went through. They will see it as different. They will jealously protect the reputation of their education and differentiate your HEC MBA from their own education. Most French companies don't even care about the MBA and recruit and groom their leaders not from the MBA level but from the Grandes Ecole level (undergraduate essentially). If you graduated, as a French national, from the Grandes Ecole route, you don't even need to get an MBA really because you have already proven yourself in the eyes of those that matter in France. Conversely, anyone who has not graduated from a Grandes Ecole is unproven! There is scepticism and mistrust of your abilities.
What I am saying is that, as foreigners, we are on a different track than locals. We don't have true access to their Grandes Ecole route. Though we don't have special access to French corporations we still do have access to multinational corporations that spans the globe. Those multinational companies have experience with the MBA and non-French credentials and would give weight to INSEAD. HEC is, as of 2015, getting a much better rep for its MBAs, but DO NOT mix up the HEC MBA reputation with the HEC Grandes Ecole reputation. They are not seen as the same things in France so we shouldn't be tricking ourselves and misinforming ourselves either. The Grandes Ecole side is separate and still overshadows the MBA for prestige. I would wager that someone from the Grandes Ecole side of HEC would have better access to job opportunities in Paris than even a Harvard MBA does. It's a different system that we have no access to. Focus on the MBA level specific reputation and as of today, INSEAD is still higher than HEC both internationally and in France for the MBA, specifically.