I've played Doom 3 on both the PC and Xbox.

As to what platform you should buy it for, that depends on a few things:

First, do you have Xbox Live? Despite the lack of mods, Doom 3 is great on Xbox for co-op and the simple multiplayer fits much better there.

Does your system fit the requirements? Doom 3 is really well optimized, and I don't know if you'd want to miss out on playing D3 at a resolution higher than your TV, with possible surround sound, some awesome modifications, etc... plus, you can play the expansion later.