I've got an 07 Si and threw on the M front Bumper. I like the Si better. It's a very quick "cruiser. " That being said it still goes around a corner in street driving (Think tail of the Dragon or Six Gap Run) as well as an M.
Until the CDV mod is done to the M it hangs with it for the first 2-3 gears. 0-60 mph its about a half sec behind the M. Considering the M is a 4.9 sec car thats not to shabby. A fully decked out Si is also much fun to drive. With DTC on and good tires/brakes it is forgiving. Paddle shifters are pretty responsive and smarter than you to prevent over-rev. Not bad considering its an auto tranny. Not much you can aftermarket on the engine bang for the buck wise. If you want 500 horse aftermarket the M is the way to go. All that will get you in the Si is a huge bill for a supercharger and a blown engine. Finally, I luv the Si wheels
See for yourself, got the old and the new:
07 Z4Si - Mods: M front Bumper, Mstache, Alarm, Hardtop Retrofit & Hardtop, mudflaps, windscreen, DaveZ GDO, Intravee II, Gaptech Remote +, Diet hamburgers and a Fat steering wheel.
03 330i - Mods: MTechII front Bumper, 125K and rollin!