I agree that you should go to LBS. HEC is great and all, but people who are not very familiar with European schools have never heard of it. LBS has a much better known name, and you never know when you need to impress someone who has not researched MBA programs in Europe.
See if you can find out what % of students end up at the firms you are interested in, check out the recruiting pages for both schools at the firms you like, heck, you can even call up the HR departments and ask which one they would prefer.
I am applying to INSEAD, among other schools, and if I am accepted there then I think I will use the last strategy to decide where it ranks in my list. The school I go to is not the goal - it is getting hired. It is a lot harder to compare European schools than it is American ones because of how many rankings and comparisons are done of American schools.