I love how people pretend they know exactly what the final version of Doom 3 will be like. They somehow know exactly how the game "feels", even long before it's released. They know how the levels will play, and they know how the monsters will act. It's like they've already played through the whole game, except it's not out yet. That said, I would choose HL2, mostly because of the online potential. Is far as I know, Doom 3 single player could be a lot more fun than HL2. HL single player wasn't all that great if you ask me, but the success of online mods has artificially risen the single player game to some Godlike level. It was good, but it wasn't nearly as good as people pretend it was.