Depends.
If I'm enjoying myself, I put up a good fight.
If we're loosing because of unfairness (I.E. unbalanced teams, hackers, general dickery from the enemy) I quit. Rage style.
My favourite battles on TF2 have always been the lost causes. I remember how we always hole up and try and take as many of the enemy MAGGOTS!!! with us.
Actually, the BEST battle I ever had was on a Payload map, where they were on the LAST corner, and we held up like it was friggin' Stalingrad. You couldn't move for bullets, a very close-run thing. We somehow held it for a good ten minutes, then WON! The next day, the first thing me and my mate (my medic) said when we met each other in the morning was "That was THE greatest gaming moment of our lives so far" and we then reminisced happily and shook our heads in disbelief.
I like loosing a hard-fought battle, it humbles the enemy