Hello phiman,
Congratulations first of all for being accepted at both of these schools; many here would dream of being in your situation right now. The way I look at it - having been accepted to both as well - is the following :
HECProsGreat consulting placement and some cool-sounding classes that focus entirely on the field (including some with MBB immersion)
Lower cost (both programme and living)
Campus feel
Decent brand, especially in Europe
Well-established programme
Possible break in the middle of the two years to intern at top firms (fairly common)
ConsDecrease in opportunities outside of France/French-speaking countries
45 min. away from Paris (either harder to enjoy the city or a painful commute every day)
Atmosphere can be a little elitist and pro-French (look up the "prepa" system)
Not as well known as LBS in Asia
Less diverse student body
LBSProsSmaller class & tight-knit community
More diverse student body
Some access to its world-class MBA (people, events, ...)
Great brand worldwide
Great location in London
Programme is ambitious and should see a rise in rankings in the coming years
ConsNew programme (OCR and administration might not be as efficient as well-established programmes)
High cost (programme cost is bad, cost of living is incredibly high)
Harder to find work in London (one year in London and very competitive labour market)
Might be overlooked due to the MBA's reputation
This leads to two concluding issues : post-degree location and price. If you want to work in France or Belgium and if you can work up some decent French, HEC is by far better than LBS. If you really want to work in Asia (or London), LBS is going to give you a leg up but it comes at a substantial cost.
As stated above, my situation is fairly similar to yours and I believe I might have picked LBS if they offered a full two-year or three-semester degree. However, playing your cards right and networking with consultants in both London and Shanghai can open many doors. A scholarship from LBS could probably tilt the balance in their favour. In other words, I'd say LBS carries higher risk and reward while Paris is fairly safe - meaning you're pretty much assured to find something decent after HEC - especially if you are young.
I hope this can help you make a decision.
Best of luck,
Ruhm