why would u ask now which platform is better if u already have your car?
to each their own but i prefer the rwd version of the 335. Best way to gauge drivetrain loss since its different on each car is to take it to a dyno and do the simple math yourself. General rule of thumb though is around 12-15% on MT cars and a little more on AT (this is not bmw specific).
2015 MW/SO M4 M-DCT [CURRENT]
2011 AW/CR 335is DCT FBO [SOLD]
2008 AW/CR 335i 6MT FBO [SOLD]