The difference is that, as I understand the US system, if you are really brilliant at MIT you will go on to get a PhD, while if you are really brilliant at Polytechnique you might very well decide to stop there (OK, after your 2 year "application school"). Which, for potential employers will translate into: "this guy does not have a PhD, hence he must not have been that good". Probably not a major problem for MIT/Polytechnique alumnis, but potentially one for lesser fortunate students ;--(