From what I heard, academically speaking HEC's Management Master is not that good. Pretty simple stuff. However the prestige in France is beyond imagination. There is no better thing in France than visiting HEC, except for Poly and Sup Normale. LBS's program is pretty young and actually not that good. They're not part of CEMS and generally the LBS MBA brand does not really rubb off to the MIM, though I am not British so in Britain perception might be different. So if u wanna do Banking in London maybe LBS is the place to go, but apart from that if u consider industry, or any job on the continent HEC might even be better suited. And dont be too focused on London. London IB = shit pay and crazy prices, so actually working in Europe might be better.