Like Dan said, I really didn’t care for either having had both. Like all BKP’s, both have that 2D, sterile, no personality/vanilla thing going on. The Aftermath is just a tighter, modern, ceramic, slightly quacky/nasal take on it (focused mainly in center and uppermids) and the Rebel Yell had a bit richer, more traditional Alnico sound, but also tight and articulate (at least something positive there lol)
The Lundgren M6 is imo a way better pickup, but also very different with a somewhat specific sound. It has big lows that are somehow tight, defined yet kind of boomy at the same time, scooped mids, strong highs. No one talks about it, but the M6 neck pickup and cleans are also some of my favorites, in some ways more impressive than the actual M6 bridge high gain tone lol
For high output pickup my real favorites are the vintage Schaller Blade, Hex neck, Dirty Fingers and after that the various vintage Bill Lawrence’s and Mighty Mite’s. No BKP belongs in the same conversation as these lol, but I do like Lundgren’s for something modern and unique