I have a Sig522 SWAT and 15-22.
You can read all about the 15-22 on this Forum so I'll just talk about the Sig in relation to the 15-22.
Sig522 SWAT
on par is more accurate and reliable with a wide array of ammo. Trigger out of the box is quite good. The Sig weighs a ton. It's a full pound heavier (without any sights) than the 15-22. The Sig stock is lower so you can use red dots and such without having to jack them up on risers. The Sig has a front sling attachment and a single point sling attachment. The Sig allows for the use of .22lr conversion mags Black Dog and CMMG... but other than that you are stuck with the configuration of the rifle. There's near zero aftermarket goodies for the Sig other than rail attachments. The Sig has a gorgeous metal quad rail but it will mark up easy compared to the bullet proof 15-22 polymer finish. The Sig has less than 3 inches of barrel protruding beyond the quad rail which gives it a great look, imo, for hanging a can off the end (hey Belt_Fed where's my can?
). Overall, the Sig522 is a more refined rifle. I posted a pick of my Sig522 SWAT here a few months ago. Most folks said they thought it was an ugly rifle.
The two rifles are very much different. In general, if you want a lightweight AR platform the 15-22 is THE pick, period. If you just want a .22 black rifle to play with there are other suitable choices, including the Ruger. I would stay away from Colt.
Here's a pic of my Sig522 SWAT right out of the box.