The title says it all really.
In terms of cost it works out to be pretty much a straight swap. Of course I am going to lose that beautiful F2 on the 135, however, I'm starting to think I will gain so much more flexibility.
My main reason for considering this is to have the extra length, but I can't decide if this is enough to convince me to sell the 135L? I know I'll not miss the 200 (not that it isn't a superb lens - I just don't use it enough).