Well the things to consider here are portability, concealability, and maneuverability. The pistol version can easily fit into a small bag such as a back pack or messenger bag, while the 556 swat with a foldable stock is shorter than most AR type rifles it is still considerably larger. The rifle ofcourse is going to give you better accuracy, but the pistol is really for close in fighting (pistol/submachine gun type ranges i would think). You are going to have the power of a rifle in a small package that can be put on a sling and worn under a jacket, probably not the most useful thing unless you are in the security/bodyguard business, but I suppose if you really wanted to you can CC it. You will also be able to get around tight corners and cover easier with the pistol version than the rifle so if it is only going to be a home defense gun the pistol may be the way to go.
I still havent decided which version I want, the swat or the p556. Personally im leaning more towards the 556swat but i think if you're not going to be shooting at stuff over 100yds away the p556 has its advantages.