Then it comes down to how much you trust Thermalright's tolerances. The Thermal Grizzy is milled so will have repeatable accuracy, and Der8auer isn't going to skimp on sloppy machining or tolerances. If the Thermalright is also milled then we're paying a premium for the name and (presumably) smaller manufacturing runs with the Grizzly. If the Thermalright is actually cast in some way then you potentially have manufacturing variations, which kinda defeats the point of this thingApparently it doesn't come with the lapping tool and that costs extra.
So who knows. Right now it's looking like you're paying for Der8auer's name and the premium applied to small manufacturing runs.