In terms of total stats:
Gravity Hammer uses energy regardless of whether it kills or not. I believe that the Hammer uses 5% energy per swing. I'm not sure if the Hammer can be used to knock around physics objects to kill opponents effectively. Regardless, it'd probably be easier to just rush and kill.
Energy Sword, on the other hand, uses 10% energy per swing, but only if it kills a target (or hits an object, like a Warthog). It cannot be used to move physics objects (for a potential kill), compared to the Gravity Hammer.
Now, in terms of range. I don't know. Someone who knows how to test ranges should probably figure that out for me. I would think the Hammer would have slightly lower range compared to the Sword, but I'm not sure.
I think Hammer swings are also slightly faster than Sword swings, but again. I'm unsure.
Based on stats alone, I'd have to for Hammer, as I can get more potential uses out of it. It also seems pretty good for a "Keep away from me" stick, causing most opponents to think twice about a Spartan Charge.