In my opinion, Doom 3 is not a game can beat HL2.
Just becuz Doom 3 has no sufficient light and no. of players in multiplayer, it makes it so easy to get bored of. However, HL2 has great multiplayer compared to Doom3. With up to max of 64 players in 1 single server is very amazing. Remember Doom 3 has only max of 4 players in 1 server. Even the expansion will come out only can max it to 8.
So you want good single player+good multiplayer or only good single player????
I hope this will help u to decide which game to buy.
My opinion is to buy HL2 ofcourse.