Power fists are sort of the catch-all cure for big, tough, scary things. Generally, you're going to wound things on 2's with a power fist, and it ignores armor saves. However, you have to have two power fists to get an extra attack for being dual-armed with one, so you're not throwing out the volume of attacks that someone with a power sword or the like will be. Also, a fist strikes last, so it's possible that the guy with the power fist will get killed before he gets to swing. Power fists are things that I personally never leave home without-- the possibility of getting in close combat with something my Marines can't hurt is quite a bit higher than people give credit for; and a power fist makes it possible to eliminate those targets if they do reach my Space Marines.
Lightning Claws are great for clearing large swathes of infantry. Their power weapon effect aside, a model using lightning claws re-rolls failed to-wound rolls. Most of the time, that means that whatever you hit with, you'll probably wound with. They're great for cutting up relatively-high toughness models in large groups-- large squads of Chaos Space Marines, Orks, and Necrons all come to mind. Just like power fists, you're only double-armed if you've got a pair of lightning claws, but generally, they're available to be taken as a pair, so you're going to be able to get the extra attack most of the time.
Both lightning claws and power fists cost about the same to field in points, and both have their definite uses. When in doubt, the power fist probably wins out, though, purely because it gives squads some much-needed extra punch against things they can't normally handle. The lightning claws are an efficiency tool, and are great on independent characters with high initiative, whereas the power fist is best left in the very capable hands of a Sergeant, as part of what you're paying for in an independent character is their better initiative!
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