This is kind of an interesting scenario, that I can sort of relate to.

How? Well because my mountain bike is about 30-31 pounds or so w/2.5" knobbies and has an 11-34 setup vs my roadbike which is about 20-21 pounds with a 12-27 (i swapped out the 12-25) cassette.

I've ridden some of the same roads on both and the feel, in terms of effort is similar now. While I had the 12-25 on the roadbike though I would have said the MTB was easier to spin on for hills & climbing.

The 12-27 swap on the roadbike though has made a big difference and I'd say that makes it easier to spin. In terms of speed, it is no contest obviously.

To see if you'd have the exact same range with the various cassettes try this calculator:

http://sheldonbrown.com/gears/

You can see the gear inches for just about any gear combination.