technically the ABF is the better engine, taller block makes for better rod angle or something? plus apparently its stronger due to being designed with SEAT motorsport in mind, cant confirm that though just what a mate told me!
In reality there isnt that much in it, just get whatever 2.0 16v block you can find you wont be disappointed! There are some differences in the heads but nothing so drastic its really worth a head swap, just leave whichever head is on there and save the head swap for a ported jobber! As for cams, the later ABF ones are arguably the best stock ones out there, there was a post on CGTI with all the cam specs of the different 16v motors. If you get a 9a head the only cam you need to swap is the inlet for an abf, or both cams for a pair of abf ones.
The later post 94ish 9a has the same zaust cam and head as the ABF, inlet still lame so just a abf inlet cam for that one. Eay way to tell if its a late head or not, if it is held on with nuts rather than bolts its a late one. Also iirc 027... is the early part number and 051... is the late.
ABF blocks though are more likely to be a bit newer than the equivilant 9a, but sayin that if you get a 9a out of a Passat its less likely to have been ragged! A SEAT ABF in theory may have been treated better as the boy racers tend to go for the Golf GTI rather than SEAT. Mine is out of a cordoba and its pretty much mint, most of the block paint is still on it and there was no corrosion on any of the waterpump/oil cooler fittings
I got mine from a breakers for £250, well matt did and delivered it to me and swapped for a huge pile of 2.0 8v gear! A good source is knackered/MOT fail SEATs, they can be had for cheap if you shop around.
edit: bore is same on both
