1. You can't use mutilate with fists weapons. Mutilate = dual daggers.
2. Mutilate is stronger now, poisons do more damage, and you don' thave to stand behind target to mutilate. I don't know exactly (and to be honest I don't care before WotLK right now) if mutilate is better than combat right now. According to spreadsheet and my gear, mutilate and combat fists builds are close in dps power. Combat is still good spec, maybe better than mutilate, maybe not but I know one thing: they are close in dps. :)
3. Stay combat for leveling. Use same build like you have right now, it will be good leveling spec. Maybe other talents would be better, maybe other spec would be better but it's just leveling. At 80 level you can change your talent build so it's really no big deal if you will put few talent points in not best talents.