I haven't written a comparison. I would say the RVKK overall is more rounded and a little deeper in flavor because the red Va. has been pressed for how ever long Sutliff does that. It's a little more earthy and lightly sweeter, lightly tangier along with a touch of fermentation that the 515 lacks.
I’m smoked a fair amount of both and for my tastes the Crumble Kake is sweeter, fuller, and has more variation in flavor down the bowl. That said, I’ve got a plug of 515 coming out of the press tonight- comparing 515, pressed 515, and Red VA Crumble Kake sounds like a fun weekend project.
If this makes any sense, I thought the 515 was more indicative of a standard red VA. The crumble cake had a lot more going on, for better or worse. I don't pretend to know what all is in the crumble cake but there's a lot more than just Virginia.
If I were looking for a substitute between those two for 5100, I would take the 515. If I wanted more depth and complexity but still the basic essence, I'd go with the crumble cake. Watch City's Simply Red is closer still to 5100 but has the depth and presence of the Crumble Cake, and is my preference between the three. But you can't go wrong with any of them, really.