Would you recommend Kitty String XL for whip and slack tricks? Or should I get the Fat version?
I would suggest the tall fat. It’s much softer than regular kitty imo, it’s super whippy but not so whippy that it over whips and it doesn’t snag or slow the spin of the yoyo near as much as the xl. Plus you have the option to go full Ricardo fraolini or Evan nagao style with the long string if you want or you can just cut it if you want regular length. Regular fat is the same string it’s just shorter and for whips and slacks sometimes I prefer to have more string to work with. The new cloud string is really good too. I don’t know that I would want it as my main string but it is really fun to play with and ridiculously whippy. It is also even longer than tall kitty I think. Hope this helps 
Get iyoyo slackies