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.
2014 LCI 550i M Sport | 3D Design Front Spoiler | BMW Performance Black Grills | BMW Performance Trunk Lip | BMW Performance Diffuser | Painted Reflectors | Wincos 30% Tint | Supersprint Exhaust | ACSSprings | MORR VS8.2 Rims 20x10R and 20x9F | Michelin PSS Tires |||More Photos of my car here|||