Whats a better processor?

[[TC]»Elmo«]

Depends which P4.

The Athlon XPs were faster than the equivalently numbered P4, so an Athlon XP 3100+ was faster than a P4 3.1GHz.

The Semperon's were slower than the Athlons, so probably roughly equivalent to the P4 or very slightly slower.
As an example, the Sempron 3100+ is about 2.5% slower than a P4 3.06GHz

If you already have the motherboard you want to use, then the brand of Processor you choose will depend entirely on what the motherboard is.