Hi,
Sounds like you are at any exciting time in terms of decisions to make etc!
I think OVERALL the best thing to do is to make the choice you think is best for YOU... both professionally and personally. The admissions decision etc will follow, and you'll be able to explain your decision well looking back if you follow what you think will be best for you long term.
With that in mind, I'd also say that HEC does appreciate international experience and exposure, so doing a year in Belgium (especially if you are an American without much international experience etc) could be a plus. Grades are also important too... but again like I said focus on what you think the benefits are to you long term in either option and what you might gain. (PERSONALLY I'm a big fan of gaining international experiences but that's just me as well

).
Otherwise I do recommend if you're able to prep for it soon, take that GMAT now to get it done and have it ready (scores last 5 years) as college students tend to do better on it on average than if they wait several years, but then get some good work experience after you graduate in prep for that MBA.
Keep us posted on questions and your decisions, and good luck!