In general, (tech report benchmarks), a 1.4Ghz P-M runs about the same
speed as a desktop 2Ghz P4 processor; a 1.7Ghz P-M runs about the same
as a 2.4Ghz P4 processor.

battery life is longer on the P-M machines however.

So:
Speed #1: desktop P4 CPU as fast as you can get it.
Battery life #1: P-M CPU in a laptop that benchmarks with long test results.