I have both. I have 2 C12s, one for home and shop. But then I bought a T15, which I've fallen in love with, and I use it for pretty much everything around the shop. I'm debating the virtues of selling the 12s and buying another 15 for the house.
That said, I did notice that the 12 is slightly more convenient during an installation. With my arm at my side, and the drill hanging from my fingers, I can get to all the adjustments (torque, speed) with my thumb. Can't do that with the T15, and it's handy when I'm holding whatever it is with the other hand, the battery is running low, and I need to downshift to get the last screw or three in place before the battery is completely ka-put.
I'm with the others on the impact thing, though. Just bought one of the lil bosch 12V max litheons... it's cute, but ballsy.
Really, though, I need to start thinking in a monogamous fashion when it comes to drills. I have my 18V DeW XRP hammerdrill for big stuff. 2 C12s, 1 T15, the lil bosch impact/ single speed driver set, and the corresponding 2 speed drill/driver, a makita 14.4 impact, and a corded milwaukee. So, that's more triggers right now than I have fingers. Ridiculous.