Decisions, Descisions...
I am not you, and you are not me. As several others have mentioned, you are already adequately set for home defense. Now, to the topic of your next purchase...
Are you a hunter? Do you spend much time shooting where a shotgun would be the chosen arm? Will you spend time shooting beyond 50 or so yards? Are you interested in precision shooting, martial arms, "tactical" exercises, etc.?
Between the handguns you already own, a shotgun, and the AR, your rifle will (currently) be the most cost-efficient firearm to keep shooting. The AR platform is very modular, and easily changed/upgraded to suit changing needs or interests.
Hoping that you will shoot frequently, thereby gaining proficiency and interest, I would suggest the AR at this time. If your main reason is simply to 'round-out' your collection, eventually owning both rifle and shotgun, then buy the shotgun now (more affordable), and use the left-over monies as a seed fund to start saving for the AR.