That's a tricky one. I have a Dell Inspiron Somewhat with a Centrino 1.4 ghz and I'm very happy with it.
I was told that you can double the ghz rate to compare it with a pentium 4. Kinda like the AMD ones, cause of the better architecture. But the again I'm not sure about this.
Keep in mind that the Centrino has built in W-LAN and Bluetooth. And I can confirm the longer battery stay. Mine runs somewhat about 4 and a half hours, which is very much.
So in my opinion, Centrino is the best for a Laptop-build.
Edit: I'll try Doom3 on mine shortly, gonna post the results.
(Centrino 1.4ghz, 768mbram, 9600pro 128mb)