I'd put AA Gold slightly above DL Silver though I've been lucky on upgrades with both. However, when you don't get an upgrade, AA Gold seems slightly better as Priority AAcess seems to mean more than Zone 2 on Delta. AA Golds always have access to AA First Class check in desks (or Business Class in three cabin airports) and priority security lines while DL Silver have separate desks at hubs and do not get access to priority security lines.