I drive a 128i vert with a manual so I guess you know what my vote will be. It depends a lot on what your definition of enough power is. If you find a 128i to drive it will very likely be an automatic. Unlike the 135i, the 128i vert with automatic is almost a second slower to 60 mph with the automatic. I suspect the coupe would be the same. So you could get a conservative feel if you can find one. I drove a 328i coupe with manual before getting my 128i vert with manual. It was the only manual I could find to drive. It was a pretty good manual but the 128i has shorter throws and feels more precise to me. In other words I think it is better. I think you would thus find the 128i transmission to shift better than your current 335i.
My 128i vert is rated at about a 6 second 0-60 and about a 15 second quarter mile. Published reports are a little better than this. I remember when "muscle cars" were often around 15 seconds in the quarter. While the 135i is decidedly faster, and has better brakes too, the 128i is not by my definition underpowered. It doesn't feel remarkably quick until you get up around 3,000 rpm and above, however. So it is more of a drivers car by my way of thinking than the 135i. If you want a 128i to move along quickly you have to have it in the right gear. The 135i has so much torque, the penalty for choosing the wrong gear is significantly reduced. When I spend time in the 3000-6000 range, which isn't often enough, I find myself challenged to keep up with the shifting and I have to pay a lot of attention to what I am doing. Speed comes up real quick. Gear changes need to be made pretty quickly. It is fun.
If you need to be able to keep up with the current V8 mustangs and cameros, you need the 135i. If you just want to keep up with the fastest current vintage family sedans and many old muscle cars, the 128i will do it nicely.
128i Convertible, MT, Alpine White, Black Top, Taupe Leatherette, Walnut, Sport
Ordered 5/22/09, Completed 6/4/09, At Port 6/9/09, On the Georgia Highway 6/13/09, Ship Arrived Charleston 6/24/09 at 10pm, PCD 7/21/09