Well, obviously it's 1 stop slower... The 90mm Elmarit M is a very nice lens, which I've used a lot. I have no direct experience with the F/2 Apo, but friends of mine have them and they are also very nice. I think you would be hard pressed to tell the difference between the 2 lenses under most circumstances, with my vote, if I had to choose going to the Elmarit. I only use black and white and the Apo is a bit too contrasty and hard edged for my taste. I think you should use your new lens without worry, I'm sure you'll be very happy.
Richard Wasserman