If you just can't swing a no compromise commuter and a decent mtb, I'd think a cross check would be be the best compromise. I say this because you should want something more tuned to your commute as that is 90% of your riding. A cross check should make a pretty good commuter and be ok off road with big tires. You might even want two sets of wheels/tires, one set for the commute, one for off road. Quick change.