Probably Doom III dude but I can't say for sure, HL2 looks REALLY REALLY good. I have a feeling there will be a little more variety in HL2's level design & gameplay then there will be in Doom3. ID software titles usually don't have alot of variety in levels (especially in Quake & Quake II)but they are getting better at this. Remember how much better Half-Life was then Quake II, inspite of its inferior engine? One would almost say HL has a better engine on first inspection, but it really doesn't. Valve was just more creative with their use of the engine. That's why I say gameplay holds more value than the graphics capabilities of an engine. I would personally rather own Half-Life than Quake 3.
Oh BTW, don't take to much offense to my post in your thread in the CPU forum, ok?
My OS features preemptive multitasking, a fully interactive command line, & support for 640K of RAM!