I understand the Kooks and QTP are similar in design, except the QTPs have the HVMC collectors. NO, they aren' t made by the same company. I know the Kooks has a velocity spike, like the standard QTPs had, a while ago, and both are T304 stainless. I understand you have tubing size options through Kooks, which I don' t think QTP offers yet. Y-pipes are a different story though. The Kooks is better, from what I have seen, BUT, the QTP has a merge collector, even though it gives up some clearance. I have modified my car, a little, for a little more ground clearance, by sectioning a piece of a stamped crossmember, in the floor, above the collector outlet. It' s only a 4"x2" piece, and I felt my SLP SFCs made up for what I cut out. I am planning on adding a piece of shaped sheetmetal to that area, later, to make up for the opening, but, I have not noticed no negative side effects. I have better clearance though.
I guess it boils down to a few things: QTP has HVMC merge collectors and a merge collector in the y-pipe unlike Kooks, Kooks y-pipe will likely have better ground clearance, BOTH are T304 SS, Kooks has more tubing size options, and QTPs are polished.
Hopefully this is helpful. I think both are comparably priced, but, I think the QTPs are a little more difficult to get, because of demand. There have been posts, lately about quality, but, I remember seeing similar posts about Kooks a while ago. Both seem to be reputable companies, and I' m sure would be happy to get you squared away.