WAAAAY more bottom end on the Nailbomb!
My off-the-cuff answer, and I think Phil will agree with me, is that the Nailbomb is more METAL and the Rebel Yell more ARENA ROCK. They both have the hallmarks of every other BK pickup I've encountered: meticulous build quality, sustain for days, and lots of clarity. I picture the NB working really well in a 2 guitar band, with another pickup adding colours to the pallette, or even another NB but with different EQ applied. The Rebel Yell wants to be the ONLY pickup you hear! Which makes it work better in 1-guitar situations, imo.
Another thing: don't sell the VHII short. It is a LOUD pickup, and it crunches AMAZINGLY well. I'll wait for Phil to chime in with the technical details, but he installed a magic switch on my Strat that tamed the high frequency peak on the guitar ... the result is nothing less than WOW.
Final note: before discovering BK, my favourite pickup was the Screamin' Demon, also a pickup designed for a single HB super strat. So, I'm biased towards that style of pickup, and I admit it. The reason I mention this is because my observations are subjective. I'm not a scientist, nor do I claim to be. However, I DO have the privelege of playing through a world-class selection of amps and guitars every week., so I think my subjective observations hold more merit than the "bedroom shredder". (In other words, my bias is well-founded :) )