I generally assume what the developers themselves say to be pretty accurate, don't you?
Hi! I'd like you to meet my friend, John Romero...
That aside, there's nothing inherently foolish with being more interested in a certain game at this point; maybe you don't dig the Doom 'horror' vibe, maybe you prefer HL2s setting, etc. That's perfectly sensible, in fact. However, that's a world apart from saying one game will be better than the other. You might have preferences and you might have expectations, but you don't have knowledge of which game will be 'better'. Sure, it's unlikely, but maybe you will find yourself totally engrossed in the Doom3 experience and find HL2 to be a lame rehash of the original game, just with improved graphics. I definitely don't think you can fairly dismiss Doom3 because previous id games have had weak single-player; that just hasn't been their focus. For Doom3, id says they're concentrating on single-player, and have hired both a sci-fi author (Costello) and an animator with cinematic experience (Nilsson) to help build the game, so at least they're taking steps to improve the single-player that they haven't in the past.
If their efforts fall flat and it's a generic shooter, well, hey, you're right. I personally expect HL2 to have much more compelling gameplay than D3 (as I don't care for John Carmack's design theories). But as I said before, we won't actually KNOW until we play the games.