I really don't understand why so many 135i owners "hate" on the 128i like it's the chump car or something. Sorry, that's the 328i as far as I'm concerned; i.e., the larger, heavier, and slower "every man" BMW
. Anyway, I digress.
If you read the (admittedly few) reviews on the 128i, they all mention that the 128i is the "purest" non-M BMW still made and in terms of driving feel, chassis balance, power/chassis balance, etc., it's better than the 135i. Of course, this is in stock form, but anything else wouldn't be an apples-to-apples comparison. I think what it comes down to is that in stock form, the N54/N55 has enough HP/Tq to overwhelm the E8X chassis and suspension. Whereas, the 128i is powerful enough, but not too much, and so it can therefore be more easily pushed and exploited.
Having had a few turbocharged cars and a few hi-po N.A. cars, I can absolutely appreciate the flexible powerbands that come with attaching hair dryers to an exhaust manifold. But, there's something about the minute incremental feel that a smooth and free-revving N.A. engine brings to the table that is very satisfying. The N54 and N55 are great engines, but they still use forced induction and you can tell. Yes, BMW has done a great job of hiding the turbo(s), but you lose the incremental feel of N.A. when you introduce turbos.
Besides, it takes a lot more skill to drive a low-HP car fast than it does a high-HP car fast