See thread238-3663 for an earlier discussion. In the current world, the relative merits of each frequency are not terribly significant. 60Hz machines and transformers require less iron in the magnetic circuit but 50 Hz systems have slightly lower inductive voltage drops.
At one time, there were quite a few 25 Hz systems in the US, and there are still a very few in service for special applications. Aircraft typically use 400 Hz for weight reduction.
