My thoughts:
TF2 can get old fast. Sure you can probably play this game longer than I did.. If I played it for only a few trial days before I got bored, you might play it for a year.. But still, you can only do so much with that game.
COH on the other hand is much deeper and sophisticated than TF2. I know it's not very smart to compare them, they are very different, but IMO there's just so much more VALUE in COH than in TF2...
In the end, I'll tell you this. If you want some quick, primitive fun for a short amount of time when you come home from work and want to vent your frustration by blowing some people up, go with TF2. It's also relatively cheap.
If you want more from a game, and you like tactics, strategy and history, go with COH. I chose COH + Opposing Fronts expansion, and I am satisfied with my choice. I might not be able to get quick jolts of fun in random matches blowing people up whenever I feel like it, but I more than compensate for it, when my friends are finally all free from work during weekends and we all gather up for one big 2 vs 2 or 3 vs 3 battle.