Personaly I would get a cassette wheel and a double-ring crank. You can run whatever two rings you want up front and have your choice of cogs in the back. If you have horizontal drops, or track ends, you can have a three or four tooth variance in total tooth count (ring+cog). If you are running a tensioner, you might have to adjust it, but you can get away with different counts, as well. My plan on the monstercross bike (also my commuter) is 42/16 (2.62) for the road and 39/19 (2.05) for the trail.