TONS of info on this head if you do a search. There is an active 'mega thread' not too far below your thread.
The short answer. Volume, reliability, quality, service... top of the heap. Fantastic.
HOWEVER, if you really dig the quite bright, upper mid present, modern sort of tone of your GK head, the fat, chewy, organic and somewhat darker Aguilar voicing might not be your thing. If you dig Duck Dunn, Bob Glaub, etc., the Aguilar is awesome. If you dig Chris Squire, Flea, or Marcus Miller, not so much (as a gross simplification)
I gigged an HD130 (the earlier version of the HD150) for a couple of years, but it was so long ago I can't comment on the comparison of the Ag to that head.
If you love your 1001 but want a micro, the MB800 (or MB500) would be the thing. If you want it more 'classic, warm, fat', the Aguilar is pretty cool.