check the other conditions.

aaisp includes vat zen excludes.
extra traffic on top is 10x the price at zen vs aaisp. - so if eg. use 160 gig at zen either have to start new contract, which starts new 12 months AND pay off the old contract as its considered terminated early. Or buy the extra 10gig as a one off which are at zens prices horrific.
aaisp £1 migration
aaisp will argue with openreach to dispute any engineer fees, zens t&c's seem to state they just automatically passed on.

on top of this aaisp also has only 6 month min term vs 12 months.