If your going for performance, the Pentium M. Its extra 256 will help you along.
Of course, you could just upgrade the ram in the Sempron... but that will cost... $100?
I'd go Sempron, upgrade to 512 corsair SO-DIMMS (Assuming it supports it... I dont know), and then sell the extra stick.
Thats me, and I just glanced. Your preference.
And dont upgrade without asking acer if you can.