The Puppetswar arms are nice, but the fists are just a better choice once everything is taken into consideration. While the lightning claws are the perfectly serviceable and cheaper option points-wise, the power fist upgrade boosts the unit's strength up to 10 and reduces the AP down to 2 making them excellent tank-hunters as well as infantry squashers, but they do strike last (granted their base Initiative of 2 means it's not really a huge downgrade). On the upside, the Ursarax' 3 wounds and T5 mean they'll be able to weather a fair bit of punishment before bringing the pain - a 27-wound unit of jump infantry potentially throwing 36 S10 AP2 hits around is nothing to sneeze at!
Sadly winter seems to be upon us so getting them based and primered is going to take a little while. Still, it's Colorado so it'll be warm and sunny soon enough! Again, all credit for the conversion idea lies with Eye of Error's fevered brain - though I'll admit cursing his name while scraping off all the aquilas and marine heraldry from the Centurion fists! Hah!