While the AT-AP would see the AT-AT first and be able to shoot first, I believe the AT-AT's armour would hold long enough for the 40 Stormtroopers or the 2 AT-ST's to disembark for battle.

Even if the AT-AT does go down, its troops or walkers would be able to mount a counter-attack. Without it's own infantry or walker support, I'd say the AT-AP ultimately loses.

JD