I agree with Biggus' point here. At this stage I think the 'cup GK' and 'league GK' labels have to go out the window. 2 games from winning a trophy here - just pick your best goalkeeper. Whether it's Chelsea away in a few weeks or Wigan at Wembley, if you're going Fabianski then you think he's the better keeper.
But I guess you could apply that same argument to the Liverpool QF game too, which shows Wenger doesn't see it that way. Wouldn't be surprised with Fabianski in goal and Sanogo up front against Wigan, and Szczesny in goal for the league games.