I loved Doom 3 to death. It just had a ton of atmosphere and awesome graphics. That's what Doom games are. Awesome graphics and tons of atmosphere. I yelled and shouted at the computer more in Doom 3 than in Half-Life 2. HL2 was a superior game, if you want to get into the "technicalities", but I was on the edge of my seat with sweaty palms much more in Doom 3. I mean come on, getting teleported to Hell? Fucking excellent. I had a grin on my face for an hour. I loved the game.
Sadly, if I had to make one purchase, I would go with Half-Life 2, for the sheer technical standpoint, but I have to agree with the other guy also. I didn't really care about Gordon or the "resistance." I was just playing it, like Doom 3. Doom 3 didn't try to make a boring plot interesting, it was just there to make you kill stuff over and over. Gotta salute that