What's the terrain like for your commute?
I'd defo be looking a commuter bike, or at least some skinny tyres on a rigid MTB frame (or lock outs for the front suspension)... makes it soo much easier and pleasurable, than suspension with nobbly tyres imo.
I've got a rigid Orange P7 that I converted for road use, with a 48T front cog and skinny tyres, goes well - although I haven't used it since moving jobs about 12 months ago
For the same amount of effort, it's not going to keep up with a road bike, but you can maintain a good average 18MPH on the flats - provided your fit enough of course.