[doublevanosrc]

I've spent some good seat time in both and basically it comes down to this:

135i: better steering feel and road feel but only on smooth pavement due to runflats. Personally I preferred the brake pedal feel on the 135 just felt more linear however keep in mind the 235i i drove only had like 200 miles on the odo.

235i: much better ride, handling and killer grip due to the adjustable suspension and pss tires. The interior is more refined and lots of extra gadgets and tech. Power delivery is very similar to the 135(n55) and braking is slightly better.

At the end of the drive my main letdowns were the steering and just that the car didn't shrink around you when driven fast like the 135. I had no problem with the seating or exhaust, it's a really really nice car if only the steering communicated more. All and all I would not rule out a 235i in my future it's sort of in a class of its own.