I have a new Stainless 580 series Mini (and previously owned an older one). The newer minis are more accurate than the much cussed older ones. The minis are rugged and neither of mine have ever jammed with any mags or any ammo. Makes a good truck/farm/security gun for me.
My buddy has an SU-16. Granted, it is the only Keltec SU-16 I have shot, but it really didn't do much for me. It jams a lot, even with different ammo and mag types (I think others have had more luck with reliability, and Keltec does have good customer service). I believe it is more accurate than the Ruger, but I can shoot the Ruger better due to the stability its increased weight provides. My buddies SU is the C model which has a stock that makes it almost impossible (for me) to shoot off the bench using my normal bench techniques. If you were going to carry the rifles extensively in the field, though, the Keltec sure is nice. My buddies straps his on his back (with stock folded) while he checks his trap line in case an incidental coyote appears.
Neither are good bench/target rifles due to Ruger's lack-luster basic accuracy and the Keltecs extreme lightness. Both are fun to shoot. Both make a good addition to a basic small-bore collection, as long as you have another accurate rifle for real target/long range varmint work.
I really would suggest finding a way to try each out before buying either.