If you've ever seen the metal clips that hold the rounds in the M1 Garand, you've seen a clip fed gun. There are also the stripper clips like the ones used to load an SKS, or the magazine of an M16/AR15.
I don't consider ghost ring sights on shotguns to be silly. They are good if you use slugs a lot. If you're just using shot, then ghost rings aren't needed though. The side saddle shell holders are a pain to use. I tried one on my Mossberg, but took it off. Maybe if it was one of the newer 4 round holders, buth the 5 round was too long to the rear. I like the stocks that hold 4 spare rounds though. It doesn't add any bulk to the gun. I kindo f like pistol gip stosk too. Not the pistol grip only, with no shoulder stock though. I mean the ones that are like those of an M16, shoulder stock with the vertical pistol grip. It's just what I'm used to.
Those Kel-Tecs look nice. Inexpensive, compact, and you can get them to use different magazines. If you get the one that uses Glock mags though, you could end up paying as much for a handful of mags as you did on the gun. Full capacity Glock 17 mags (17 rounds) can cost over a hundred bucks if new, or in like new condition. But it is a nice option of have. I thought about the .40 S&W version, but I figure, if I want a rifle (carbine) sized gun, I want a rifle (or carbine) cartridge. The new .223/5.56 looks nice.