honestly, for only a couple more pts, i'd go the thunder hammer over a power fist on an IC because of the stun effect...
it's usually very dangerous for IC's to strike last in assaults (as they can be targeted!) so being at a disadvantage for only 1 round can be a life saver!
it's just too bad you don't get a thunder hammer on the commander sprues!

though you can bit order a good one from the iron hands box set, or else just use a spare plastic termie one!
cheers!