Kitty - good string, plays like a good string should, not gimmicky. Downside is it doesn’t last very long, but that’s expected from cheap bulk strings.
Fat Kitty - same as regular, difference in thickness is very minute and you won’t see the difference simply by looking at it. When you put it on the yoyo it will be a little grippier and tighter binds than regular kitty. Very tiny increase in whips, not really noticeable unless your really compare side to side.
Nylon - Superior to regular kitty in every way except softness. Noticeably better whips, slacks, and longevity. Not as soft, stiffer, slick feel.
Fat Nylon - Again, not a noticeable difference in thickness but you’ll probably be able to tell when you do binds. Whips a tad better then regular nylon.
Another thing to mention. Regular kitty is much brighter then nylon kitty