S&W J-frame vs. Kel-Tec P3AT: Size
The Kel-Tec has the clear edge in size, being not only shorter than the J-frame but also significantly thinner and compact.
From last week, the Kel-Tec weighs in at a scant 11.2 ounces (thanks for the correction Bob!):
S&W J-frame vs. Kel-Tec P3AT: Weight P3AT
The surprise, though, is the loaded J-frame:
S&W J-frame vs. Kel-Tec P3AT: Weight 360PD
13.9 ounces, only 2.7 ounces more than the P3AT. Heck, it's lighter than the .25ACP Colt Jr.!
S&W J-frame vs. Kel-Tec P3AT: Ammo
Here the P3AT has a slight edge, 6+1 rounds to 5. If this were a .327 Federal Magnum revolver with 6 rounds it'd be a tad closer, but then it would only be the Snubbie from Heck™, which would be considerably less impressive.
MSRP for the 360PD is a hefty $988, whereas the P3AT has an MSRP of $324. Both sell for less than MSRP in the real world, but the 360 is still commanding $650+ off the shelf (I think I saw $649 at the local gun shop for a 340PD) whereas the P3AT can be had for under $300 in most places.
So there you have it. The P3AT is the clear winner in all categories, coming in smaller, lighter, and less expensive than the J-frame, as well as offering greater ammo capacity. The advantage of the J-frame, of course, is the significantly more powerful .357 Magnum chambering as well as the more durable wheelgun action - both guns are likely to be carried in a pocket, and the revolver is less likely to be affected by the accumulated garf that resides in our pockets.
Both are a handful to shoot - the Snubbie from Hell™ obviously doling more punishment to the hands and wrists of the shooter than the P3AT, but the tiny .380ACP isn't exactly a pussycat to shoot either. For shooting economy the Smith has the edge, in that it can shoot .38 Special wadcuttters all day, whereas the .380ACP of the P3AT has proven more expensive and rare in recent times.
Neither gun is meant to be a primary arm; rather they're more back-up guns or guns to carry when you can't conceal a primary arm. In this capacity, either makes a fine choice. The Smith might have a slight edge in durability and flexibility for shooting; the P3AT is the clear leader in size and price. My own personal experience - and some will tell you I'm no harbinger of performance (ahem) - is that the P3AT is less accurate than the J-frame; however neither is meant as a long-distance weapon.
So there's the comparison of the Smith & Wesson exotic J-frame with the über-light and teeny P3AT!
That is all.