I have both and can tell you not to compare the two.
The Kel Tec is a good little rifle but not an AR. The Kel Tec has a composite receiver and while it shoots great I would not trust it's longevity in hostile conditions. Also the cleaning of the Kel Tec is no snap either. The bolt assembly is very finicky about removal and reinstallation. And the design of the receiver does not allow a bore brush to be used on the chamber. I had to bend a cleaning rod to even get a cleaning patch in the chamber for basic clean and oil. I spend as long or longer to clean the SU as I do my AR, and since I clean after I shoot...
Price wise the Kel Tec is about 4 to 500, but an AR can be built for about 650 so if I could only have one I would save for the AR. I got the Kel Tec to carry so if Calif LEO stop me I don't have to worry about my AR being gun grabbed. To me the Kel Tec is expendable.
Better to compare the SU to the Mini-14 and then I think the 14 would win.