The two cars can't be anymore different.
Big vs. small
Quiet cruiser vs. loud canyon carver
4100lbs vs 3600lbs?
I use to have a modded up 335Msport 6MT and now a 535xi Msport SAT. Two very very different cars for different purposes. It comes down to what you need/want from a car.
My F10 handles decently well for that size car but make no mistake you aren't going to win any auto crossing events with it no matter what you do to it. If comfort, size and refinement is your thing then choose the F10. If you want something very sporty, nimble and fun to drive then go with the IS.
Mineral White/Mugello 2016 F85 X5M