I owned a 135i before my M3 and have had 2 335i's (currently own one now as well). The 135i is so much more fun to drive than the 335i. And I've had 2 full grown adults in the back of my 135i with no issues. My brother is 6'1 and he fit back there just fine. Sure it was tight, but definitely doable. Just go test drive them both and come to your own conclusion. There's a lot of bias on the forums and comments from those who have not had enough seat time in each of the cars.
E92 M3 sold. S2000 acquired. Cayman GT4 allocation obtained.