I had a E90 335i sport with manual trans. and sport pkg a few years back and got bored of it quickly. Added torque is nice, but the engine falls flat on its face quickly. The M3 is a completely different animal. It really depends what your priorities are. If you don't care about pushing that screaming V8 to 8400 rpm but would rather have higher mpg and a nice comfy ride go ahead and move to the 335i.
I also don't think that the E9x interior is dated, in fact I think that the F30 interior looks rather cheap in comparison, reminds me of a Honda. HUD would be nice though, really digging it in our X5.
2015 M3 6MT Stripper | Yas Marina Blue, Cloth, CF | Adaptive M Susp, 437M
2013 X5 xDrive50i | Carbon Black | M Sport + M Perf, 333M Wheels