Over the years I gradually ended up with a 50,80,150,250,350,2x and some closeup tubes. While I generally find a use for each, I think for a two lens outfit I would go with the 50 and 150. When traveling those two lenses plus the 2x cover most of my bases. When going out locally I take what I feel like using at the time. What you might consider is a second body that way you don't have to change lenses or backs if you want to use two same film (or different films - just change the lens).

Having recently acquired a Titan pinhole the lens decision is greatly simplified -- no lens to bother with.

http://www.jeffreyglasser.com/