You can be very effective with both. There really is no one magic platform that is better than the other. Both can be configured to maximize your capabilities. It's your personal talent, execution and style that will make the difference.
1. If you've seen P4 slower than P2, something's wrong with P4.
2. CPU speed is not critical in a trading computer for speed of execution... P3 = P4.
3. You might choose P4 if you're doing heavy duty number crunching, but i doubt you'd notice any dif.
4. Your "pathway to execution" (dial-up, cable modem, etc) is much more important than CPU.
Of all the processes in the trading "chain of events", CPU speed probably isn't even in the top 10. #1 is the trader's angst over pulling the trigger.
Want to economize? Build an 800mhz PIII with 512mb of Ram on Win2000 + cable modem. The dif between this and THE fastest is probably measured in milli-seconds.... it will take you longer than that to swallow before you hit the key.