Never shot the Nano, but as has been mentioned before, the lack of a slide stop and feeling of the grip weren't what I consider comfortable considering the size. Also, The top-heaviness of the pistol felt a little awkward but I imagine that after loading up the magazine that would change.
Shot the Ruger simply out of respect. I've never had a bad experience with a Ruger except for a busted extractor in my P95 that the company quickly rectified. It felt decent in the hand and the size and "melt" really make it ideal for carry. However, I just couldn't get past the trigger. I didn't like the return, I didn't like the break, if you could call it that. It wasn't a bad handgun for the price, I just didn't care for it enough to drop $350 or so.
I liked the Kahr from the get go, except for the short frame. I have fairly wide hands, and average length fingers. I'm sure a Pearce grip would remedy the uncomfortability of the grip frame, but I loved the trigger and sights on a stainless pistol as small as these come. I ended up with the CW9 for a little more than a song and couldn't be happier. Like others have noted, it came out of the box with a better than average trigger and the only thing I would change is the size/shape of the trigger guard to accomodate a slightly larger finger. It really was almost a toss-up between the Kahr and the Ruger but the trigger made all the difference to me.