I'm partial to the KB. Their 2.8 mammoth kit is a monster and makes the same power as the Whipple 3.4 Crusher plus fitting under the stock hood.
Jim Bell is a really good guy too. I've seen him talk about both blowers and he'll openly admit if something was wrong with his own. Everyone I know has bad amazing experiences with KB about parts and questions, including rush delivered replacement parts or missing parts at company expense. Their kits, unlike Whipple, include supporting parts, like a BAP.
When the time comes for me, I'm going with KB hands down.