I hate to rain on anyone's parade, but if a total conversion is going to be made, for *any* game, that requires a team of dedicated and knowledge individuals. Before you start thinking of doing something like this, a few skilled moddelers ought to be onboard for it. Thats the part which can be done prior to the release of the chosen game.
So if you think you have a few guys that can model well, and they're in it for the long run, than now is the time to start producing models and textures. Of course, if this goes ahead, when the game (HL2 or D3 from what I'm hearing) comes out, then you will also need coders and level designers. There seems to be no shortage of the former, but I'm not so certain about the latter.
With that said, I'm hoping that this can be made a reality. The problem with the RF engine was the incredible lack of modability. In this respect, I think that HL2 will far exceed Doom3. The Source engine seems to be engineered from the ground up to be totally modable. We all know how far the original HalfLife engine came in terms of mods, and if the new one is superior, as it seems to be, than the modding process should be as painfree as its likely to get in the next few years.
Both engines should easily be able to accomdate whatever you throw at them, so its more of a question of Mercedes vs BMW at this point.