For long term use, the lighter tomahawk doesn't wear your arm out as much. And the hawk is easier to maneuver during a swing...it doesn't hit as hard as a heavier hatchet though.
Overall I think a Tomahawk is lighter and more precise than an axe or hatchet. It's like comparing a 4" Mora to a 7" Becker...both are great outdoors knives, but you need to use slightly different techniques to ge the same job done.