Looking for some opinions. I am looking for a home defense ammo for a Ruger Security Six revolver with I believe has a 6" barrel. Would Hornady Critical Defense or Critical Duty be the best choice? Caliber is .357 Magnum.
That said, I would not go with Critical Defense for several reasons. I really like Buffalo Bore's 125 grain Barnes XPB low recoil/short barrel load (https://www.buffalobore.com/index.php?l=product_detail&p=276). Their "low recoil" version is more stout than most full-house loads and it is low flash. In a full-sized steel revolver, the follow up shots come easily. I've run it in my Colt Trooper and it is by far my favorite choice for a home defense load in .357 mag.
Personally, I would go with your standard JHP like Speer Gold Dot.
I love Gold Dot ammo in .45. I can get match grade accuracy out of it and they expand well. I shoot a lot of Gold Dot and it performs better than Black Hills 200gr semi wadcutters. I'm sure their .357 load is just as good.