Both are great basses. While I haven't played an ABZ (that I can remember), I do have a couple of AB's (an ABI and older ABII with no finish, or is it a satin finish... a non-poly finish).
The Combustion definitely has a thicker neck back to front, more-so on the 2nd generation batches, than the ABs. I think that the Combustion is just more traditional sounding overall. The ABs have pretty hot pickups and you'd think they are active and are very powerful and authoritative, but have clarity.
If the ABZs are anything like my older ABII, the wood just has a sexy and intimate feel to it. The AB line has an overall feeling of higher quality and fit and finish, which is not to say the Combustion feels cheap; they're just not equal in that regard.
One of the good things about the Combustion is that it can serve as a platform for modifications. You can "upgrade" the pickups to FD-3's or change the preamp later if you want to fiddle with things.
Would I pay the extra for the ABZ? Yes, if I decided I liked it better. But both are good choices.
TB Dingwall Club Member #11