Portal was very, very good, don't get me wrong
But when it's over, it's over. You can replay and do all the challanges and refine your skills until you can better the insane Portal speedruns on youtube (ridiculous speed, not healthy at all)
TF2, however, keeps going, and every day is a new session with new people using the maps (which might be new depending on custom maps) in new ways, with new outcomes, new tactics, new teammates and...new friends
D'awwwww