Coilpacks tend to be "weaker" on the AUM (i.e. they fail more often). Also the AUM has a secondary air injection (SAI) system which can be prone to faults, and also has a more complex breather pipe system for the oil vapour but i'd recommend stripping this all out out and fitting a catch can anyway.
Other than that you get the advantage of having a KO3S already bolted on and a stage 1 remap should see you right to 210bhp.
The AGU has a better flowed head IIRC than the AUM but this won't come into play until you start going beyond stage 2 mods IMO.
I drive an AUM and have no other golf to compare it to but these are the thoughts off the top of my head from being a member on here for the past couple of years.
At the end of the day i would buy whatever comes along thats in good nick with decent milage etc but if you go to look at an AGU bear in mind budgeting for the KO3S conversion on top of the buying price.
HTH