I own both, but I can't decide which one to play.

Good for a little mindless few hours of multiplayer

it must be MW3, but both games will have enough players to last until the end of your console's lifespan

i'm pretty curious as to what would happen if i popped world at war in. would i travel to 1944?

MW3 but both have a ton of players.

I'd guess MW3 has more players, but there is still a healthy community on Black Ops. I have jumped back in to Black Ops sporadically over the summer and there are still a lot of players and games going on.

Even MW2 still had a decent number of players last time I checked.