I say IS-F but then again im biased
LOL joke it's basically to the owner/potential buyers preference.
I personally have driven both and i currently drive a Lexus but if its not an M, i wouldn't consider it because i used to work at BMW and now im currently at Lexus... Let me tell you Lexus cars are alot more reliable than BMW's... (Non M might i add)
But i honestly don't know what i would go for when its time for me to upgrade to either the F or the M3...
BTW, the IS-F comes with an 8 speed transmission and supposedly shifts at a .01/second rate