I've got the Nano AND P290RS. Both have, for me, many good points, and few bad ones. I'll keep and carry them both.
I think the Nano trigger is better. I like the night sights on the P290RS, and will add them to the Nano as soon as I can get my hands on them.
In my Docker style pants, the P290 comes out of the pocket easier than the Nano, and prints less. But both work fine in my pocket.
I've got closing in on 400 rounds through the Nano, and have not had a malfunction of any type. This is firing strong-hand, weak-hand, and two-handed.
I've only got 114 rounds through the P290RS. Had on failure to feed on round 24 with a Corbon 115 grain +P cartridge. Everything else was good.
Both are built like little tanks.
I had a Kahr PM9, and while it was flawless, and the trigger great, I like a little more weight when shooting hot loads, and the Nano and P290RS provide that.
Incidentally, for those that have trouble seating the 8-round P290 mag, push on the base plate, not the spacer, and it'll click in place very easily. When you push on the spacer, you're not getting full movement of the magazine itself. Try it, you'll be amazed.
I can't take credit for this find, I read it in a post on the SigForum after having my P290RS for just a day.
And I agree with those who consider the Shield in the next category up in size. That big, and I'd be carrying it other than pocket style, and would just as soon carry something with more capacity, like my G27, or PX4SC, or M&PC.