I've just gotten in to a 125i M Sport. Ex demo (4000km) for $62k with M brakes, nav, sunroof and the 8-speed ZF. (Sylvania BMW).
My father-in-law owns a 2012 135i TCT convertible - which I drive often. I've spent a bit of time comparing the two cars so this might be slightly over detailed :P
First thing I have to point out is that both of us think the interior quality in the F20 is FAR better than the coupe. Layout, features, refinement, storage... much much better. Driving position is excellent.
If I recall correctly, the 09' 135i has the Steptronic and not the TCT. Comparing the two transmissions (he also owned an 08' 135i) the new ZF is better... it really needs to be experienced to understand how great it is when you're giving it. I might also prefer it over the TCT as it's also easier to live with.
I won't comment on power as it's subjective, and I'm sure you're well aware of how great the 135i is
Like AusF20 says, a 6.0 time of 0-100 seems about right. That for me is quick. With a JB tune you'd probably be looking at 5.5? There is no 128i F20 - that 6.4 you quoted is for the Coupe.
In regards to the sound... the 135i oozes character, whereas the 125i leaves a lot to be desired in my opinion - and that's my biggest gripe. I was expecting the 2.0 Turbo to produce a much more aggressive sound under load than the 1.6 in my previous R56 Cooper S; it doesn't. Putting it in to Sport mode livens things up but it's more induction noise than a growl from the rear end. The engine sounds great from inside the cabin, but if you're wanting to turn heads with a screaming exhaust then you'd have to look at aftermarket options.
Hope that helps...