The Striker is a superior shotgun in my opinion. The AA-12's ammunition capacity is pathetic, especially when it's fully automatic and eats through ammo like the cookie monster. Sure, you'll get a couple of kills with it at close range thanks to the fire rate but then you have to pick something else up since you'll be out of ammo in no time. The Striker fires as fast as you pull the trigger, so you can conserve you ammo easier and have the option to fire slow and steady with high ammo capacity to boot.
I think the only downside to the Striker is that once you have emptied your magazine, the reload process takes a while since the weapon houses roughly 12 shots per mag. Aside from that however, it's a great shotgun and I prefer using it over the SPAS12 because the pump action on that actually gets you killed. When you fire with the SPAS12 it's easy to go into a sprint straight after your kill. What you don't realise is that you didn't let the shotgun pump the old cartridge out, so when you stop to shoot another player your shotgun begins that process before firing so you end up getting killed.