Weapon-wise, I don't think it would matter all that much between a spike and a hammer poll.
Yeah a spike will puncture and rip flesh on the follow through. It would definitely puncture a skull too.
But you wouldn't want to be hit by the hammer poll either. It won't puncture... but it will surely shatter bones on impact... and a solid blow to the skull would most likely be lethal.
In the middle ages, there was a weapon which combined both a spike
and a hammer poll - the warhammer.
I wouldn't enjoy being struck by either business end of one of those things.
That leaves the remaining difference a choice between usage as a tool. Spike or hammer? Which is the better tool?
I have and have used both.
The hammer poll is VERY useful. It's a hammer with about the same sized striking-face as a standard claw hammer, and it can do any hammering job a claw hammer can do.
The spike it also a useful too. It's excellent for puncturing cans of juice, or whatever canned fluid you might take camping. It's my tool-of-choice in the kitchen for puncturing cans of tomato juice when I make chili. You can of course do this with the 'hook' at the bottom of a tomahawk's bit, but it's a little trickier. The spike does it with ease. The spike is very good at chipping ice in the winter time. You can also sink the spike into a log so you can use both hands to hold large material you need to slice by sliding it over the 'hawk's bit. The spike also can be used as a pick for digging in hard soil.
). The hammer makes more sense as a general tool.
