Rebel Yell is bright, mids, and a beautiful harmonic sheen. Tonnes of clear, open crunch. It's a very 'consonant' sounding pickup.
The A-Bomb is hairy, angry, and more 'dissonant sounding. They're more the same than different, but the A-Bomb is warmer, thicker, more gnarly, and the pickup tends to have a bit phatter of a bottom with more emphasis in the treble instead of the upper mids and harmonics. It's a great sounding pickup but I'm curious how you will jive with it.
Incidentally, Rebel Yell Bridge / VHII neck combo is phenomenal. I love it!