Found this on Chris' OCS list geeklist:
Sorry for the delay in getting back to you on the EFS vs. OCS thing, my access to BGG has been sporadic of late as the site has been blocked.
I like both EFS and OCS a lot. EFS tries to do less (much more abstract airpower and supply, and few naval rules), and so does it's thing better. I also think it does a much better job of showing the command difficulties the Soviets operated under with the HQ rules and limitations, and the different sequence of play the Soviets have to use. But it's also a lot less flexible - you can play only '41 on the eastern front.
The systemic advantages EFS has are substantial, and the fact that it has more and more interesting small scenarios would seem to seal the deal. But there are a few chinks in the armor too - OCS's combat is more interesting and exciting, and its troop quaility ratings make for a more nuanced system. And you can play Burma, Korea, Tunisia, DAK ... a much wider and more interesting set of situations (although admittedly vonBorries used a system very similar to EFS in his great Kasserine and Roads to Leningrad games).
I like them both a lot. I've ended up playing GMT's Kasserine more, though, because it is basically EFS but it "borrows" some of the good stuff from OCS, the scenarios are smaller and more manageable, and I find I have less time for monster OCS games of late. I really enjoyed them the few years I was into it, but I have a hard time seeing how I would ever manage to find the time to play EatG, GBII, or Hube's Pocket again. Hopefully I can get a chance to do Korea or Burma, though; I've played campaigns of both and they are managable and thoroghly enjoyable.