Under your scenario you're right, but I think your assumption would prove wrong. I think that naturally the number of lefthanded AB's is roughly equal to the righthanded ones. However, if your pitcher was weak against lefties he's probably see 70% or more lefthanders as managers move to counter his strength.

I'd take the pitcher who is better overall at getting people out. Pitchers who are dramatically weak against one side, just like hitters with a large platoon differential, can and will be exploited.