I have owned and /or test fired many of these guns. Size matters. The ones with the tiniest grip are awkward to aim and fire. This makes them either really slow, really inaccurate, or both. I like the "Black Widow" model, which has real sights and a medium-sized rubber grip just a bit shorter than you'd find on a J-frame S&W revolver or a Kel-Tec P32 / P3AT.
I like the magnum caliber for defense, because of the extra noise and brilliant muzzle flash. If I shoot at somebody, they're going to think they're being fired on by a .357 magnum! But since most of the gunpowder burns in the air outside of the barrel, the magnum bullet doesn't really fly too much faster out of that little barrel. And don't expect it to expand. It will just penetrate an inch farther than the .22 LR would.
I used to see the .22 LR small size NAA mini-guns used, at pawn shops, for $100 -$150 all the time. Now the price of a used one seems to be closer to $180, and new ones have also nearly doubled in price in the last 15 years or so since I first started paying attention to them.
How far are you willing to drive, if I find one at a fair price?