I commuted regularly on a MTB for a long time. Just put proper slicks on both bikes, something that can support a high pressure, and it should be "good enough." Also, adjust your suspension to be as stiff as it'll get, to keep it from eating too much energy.

There are differences in gearing, but for regular road riding, I generally use a very high gear - usually the highest or second highest - on an MTB and a middle gear on my road bike. I haven't done tooth counts, let alone corrected for the different wheel diameter, but I'd guess that the gears I actually use on a regular basis are the same. Also, if you find you want higher gearing, replacing your cassette or chain ring is much cheaper than buying a new bike.