as tom said the hike mode is a big difference, also the QSTs are substantially lighter than the xpros, not sure if that made a difference.
I haven't skied the QSTs but trying them on i found them to be fair bit lower volume, width seemed pretty similar but they felt lower over my instep for sure, shouldnt be too much of an issue if you take them to a bootfitter the bootboard can always be ground down.
the fact that the main thing you care about is how they ski in off piste conditions means baisically any of the top end salomon line up will be good (assuming fit is good), alot of the fwt guys are skiing raceboots as they give the best downhill performance.