It's different for every map, but overall I would say Red. I like being defensive on maps and so I get to play Engineer and Heavy more often. If I'm in the mood to Pyro or be a Spy, then I'm on Blu.
I personally enjoy blue team because even if I'm on red I shoot at red and then realize "oh....I'm on red....".....red just makes you want to attack them.
personally, I dont mind xD I use auto assign all the time, its much fairer if im honest, I even use auto assign when I play CS/CS:S its just how it goes for me! hah xxx