The ABA, at least in my area(and probably most), is way more plentiful than
the 3A.
The ABA has longer rods for a better rod ratio and presumably smoother
running.
ABA also has oil squirters, and a forged crank, at least on the OBD1 models.

3A's came in the Audi 80.  Maybe some other models, but I can't think of
any.
I put a 3A in my friends MK2 GTI and will admit that it runs great.  Just
not easy to find them.
Some day he's going G60 turbo, so I'll get that 3A back and offer it if
anybody wants it.