I think most here will say TTRS, without a doubt.
First, the things that sway me are the uniqueness of the car, shape and rarity, especially being an RS model. You don't see a lot around. The M3, although nice, still looks like EVERY other BMW on the road.
Second, the all wheel drive. We get pretty bad winters around here and having all wheel drive with a proper winter tire setup makes this thing pretty amazing year round. I don't have to worry about snow all too much.
Third, power and fuel economy. I'm getting 28-29 mpg while driving long distances; however, that doesn't mean I sacrifice power. My car was just tuned yesterday and with a simple stage 1 tune you can expect between 400 and 430 HP and ft lbs of torque depending on the tune and car... which in turn, makes these things into rockets (they already are).
Yes the M3 is nice and I would never pass up the opportunity to own one if I could, but since I don't have enough cash for two cars, TTRS it is.