RE: How many games are you playing?
I have played EFS AGC, AGS, AGN and Typhoon. I am selling all my EFS games and have just got in to Case Blue and DAKII. IMO they are superior. But as Marquo says OCS is big on logistics. But I find that in may games Pz Div's roam all over the place with no regard to the limited fuel available. OCS simply does not allow this. You need fuel to move anything motorized. EFS breaks down here. So does PzC. This is why in Uranus the Germans can check the Southern Soviet pincer with a massing of German mechanized forces, that simply was not possible logistically in reality.
So I guess it depends on how you want to play it. If you really want to see how military units are limited by supply go for OCS. If you want a simpler game then go for EFS.
Another point is EFS just does the Barbarossa period. Case Blue and GBII cover a more evenly balanced period of the war (late 41-42- early 43). In EFS being soviet is not much fun. If EFS is to expand into the 42-43 period I suspect the model will breakdown and a major rules update will be required.
In OCS there is one set of standard rules that are used for all there campaigns that range across Europe, not just Russia. Finally OCS is a much more popular game with gamers in general if you use the activity at Consim world as a guide. It has about 3 times as many posts as EFS. So you may have a better chance of finding an opponent with OSC if thats a factor.
Both game systems are good. Just a question of taste. For me its OCS.