My P3AT
The "what is the difference" question is probably best posted on the KTOG website or KTRange. the ones I can think of are...the top of the slide is flat on the 2nd gen, the sights, if you can call them that are smaller, and the extractor is bigger. I think there is one less part also in front under the barrel by the recoil spring too.
My 2nd generation P3AT is only a few months old. I got it only had it 3 days, disassembled and cleaned/lubed and put 80 rounds thru it....then the slide seized. I had failure to eject and failure to feed problem with every magazine. I did the fluff and buff with 600grit sandpaper after 50 rounds, even polished the feed ramp and chamber with a Dremel. But afer putting another 30 rounds thru, the slide problem surfaced. So, it went back to Kel-Tec, they replaced the barrel and slide, did the F&B themselves, and test fired with ammo I specified. I got the gun back in less than three weeks, customer service was great and the gun has been 100% since it's return. At the range I used Remington, Wolf or Independant FMJ with no problems. I've tested and carry Federal Hydro-Shocks though. Hydro-Shocks are the ammo I asked Kel-Tec to test the gun with and they claim they did and performed whatever repairs necesary to make sure they'll feed properly and work when they have to.
If you get a 2nd generation and have any problems, Kel-Tec does a great job of getting things corrected. There is alot of info out there for P3AT owners and there is a gunsmith, I think he's in Texas, named Fuselier that does wonders with Kel-Tecs, including some cool plating.