I bought the Ruger for the following reasons:
1. Build quality and fit and finish: I bought my LCP from a friends gun shop. He had two LCPs and a dozen or so P3ATs in stock. He let get out every gun, strip them, and give them all a pretty thorough one over. Both of the LCPs had better fit and finish and simply appeared to be better built than any of the Kel Tecs.
It is far from scientific but it seems I have much less discussion of the need to "fluff and buff" the LCP versus the KT. It wasn't the biggest sample size but comparing guns in the shop that day (and based on the other Ruger and Kel tec products I own/ have owned) I can see why.
2. Slide stop: While it does not hold the slide open after the last round is fired this is nevertheless a feature that sets the LCP apart. If one needs to clear a double feed the first thing one does, after identifying the malfunction, is to lock the slide to relieve pressure so one can strip the magazine and then clear the chamber. Locking the slide back is much harder with no slide stop. In sum, clearing a double feed may be easier with the Ruger. The slide stop is nice to have for other reasons as well.
Either one could probably fill the role they are built to fill. However, I believe the Ruger, being priced fairly close to the P3AT, is the better buy.
I will suggest, based on my experience with my LCP and my KTs, that either way you get the chrome slide. My LCP which has been in my pocket nearly every day for a number of years started to have wear marks on the slide after a couple years. I will say that I have put in a lot of practice draws from my pocket holster, more than most people I'd imagine. However, it is many fewer than with my other carry guns, which don't show the same wear. I have also experienced wear marks on the bluing of my KTs after relatively little use. If I could go back I'd get the hard chrome slide, apart from that I am very happy with my LCP.
They're the same gun. Buy the least costly of the two.
I believe it would be more accurate to say: They are basically the same design. How well that design is executed when it is actually built makes a difference.