Since I own one of each...I can speak of my personal impressions.
NNA Guardian: Great quality, decent trigger, utterly reliable. Shoots ball or HP. Slightly chunky, kinda heavy, few sharp edges traditional mag release eats up a little room under the trigger guard. Needed stippling on the front strap. Don't want to carry HP in it because of Rim-lock possibility. Sights are a joke.
Seecamp: thinner and lighter than the Guardian. Better grip, the heel mag release lets your second finger go a little higher on the grip. Won't shoot ball, but, 100% reliable with Silvertips or Speer HP's. Very good workmanship. A classic. No sight to make a joke about.
Kel-Tec P32: Mine was very reliable. A few horror stories from others. Superlight wieght. Feels kinda "cheap" to me, but, the only game in town at the weight. I really can't fault mine, but, I always kind of have my doubts because of all the problems I've heard about. Sights are as good or better than the NAA.
Kel-Tec P3AT: Mine is not reliable yet. IF it can work well, it will be the ultimate super hideout gun. Mine has had feeding problems, ejection problems and some wierd thing where the trigger will not reset, but, always fixes itself before I can figure out what its doing. It will have to go at least 200 rounds without a single burp before I can trust it.
Walther PPK and PP: Outstanding quality, reliability and workmanship. Accuracy is superb. For its weight and size, I can just as easily carry a Kahr MK9 though. (I will never get rid of the Walthers though, they are too classic. My wife keeps a PP in .32 as a home defense gun. She shoots it well, why mess with success)
In my back pocket right now is the Guardian with SB ball. I know it will work. Its a good compromise. Second choice would be the Seecamp, but, its a very early version and I don't want to beat it up.