Glad to hear you are doing great, I am new to this sight, and came upon your post.
My 2 cents, go with an AR 15. You have way more options to convert with. I have Three uppers for mine, 10.5, 16, and 20 inch heavy barrel. Getting ready to change upper on the 20 inch to a flat top so I can install a Vortex
1x4x24 viper PST CQB scope. Covers all type of situations from Close Quarter Combat, to long range. Check out AR15.com for more info.
Shotguns have their place for CQB, and packs a punch, I love hearing the sound of a shotgun racking.
I have an 590 as well, been looking at making it SBS, cost is about $500.00 that includes the AFT Tax stamp
If you decide that you want a shotgun I would look at the Benelli M1 super 90I have one with a 14 inch barrel, no kick, and my wife loves to shoot it.
You got the shotgun, if you have a pistol, get the AR and you can do 3 gun competition.
hope this helps,