It's a balancing act. Against 99% of opponents, lightning claw armed terminators are just as tough and do more damage than those with thunder hammers. The moment you come up against 2+ saves, AP 2 weapons, or T8/AV 11 opponents, they are sudenly little more than cheerleaders and extra wounds...
I would always advocate one or two thunder hammers in a lightning claw squad. Do you need it the other way around? Maybe not but that many shred power weapon attacks in initiative order may reduce the amount of damage you have to deal with quite a bit (by mauling units like lychguard or nobz before they attack).