The Hammerhead has more abilities and speed, so I like it more, but it is waaaaaaaay too weak. Reverend is correct, a few shots from a Geth Trooper and you've got alarms going off and fires are soon to come. I hope they keep the Hammerhead for ME3, but make it a much tougher, so it doesn't blow up so easily.
Also, give it an independently moving-turret, get rid of those silly LEDs on the back, and give it wheels so it isn't a GI Joe toy anymore.
Oh shit, I'm describing the Mako, aren't I...?