At stock, the PhenomII will have an advantage in single and dual threaded applications and programs that don't use 4 cores. For the few programs that actually do use 4 cores, I'd say the AthlonIIx4 has a slight advantage.
If there is only a $4 difference, I'd probably get the PII x3 and try to unlock the 4th core. If not, the PIIs are good overclockers. (same with AthlonIIs)