Also, be aware that the Longhammer is not in any weapon group officially. If you are relying on fighter weapon training, double check with you GM that they will allow it to fit in the hammers or polearm group.
The longhammer does .5 extra damage on average (not counting criticals). Keen edge and the keen weapon property do not work on blunt weapons (nor a whetstone but that only lasts a hit and takes 15 minutes to apply). The slight edge in damage goes away when you increase your size.