this weekend, azvinnie (livefast),spunout (kings), and myself (thatsdax/howell) all went for an approx 40mi run at high noon in the desert heat (it hit 104 that day). http://www.motoredbikes.com/viewtopic.php?t=1688
all engines worked very hard and no-one had any problems
... therefore, it seems that the engines are approx the same but what matters is the seller.
if you search the forum, you can get info about most sellers , then it's up to you where you spend your hard earned cash.
the "which one" topic almost always goes straight into direct recommendations, and even a lil bit of debate, but it never seems to resolve itself.
take the time to have a read around the place, including our "resources" page. there's a lot of pros and cons posted about various kits and styles. pay particular attention to seller rep and "feedback" & then tell us...what do you think you want?
ask as many questions as you need to in this thread, & your build will take shape.