I'm no expert on Karting, nor am I the fastest sim racer (not even close!), but I do own all three (KRPro, KK and Kartsim for rF2).
They all have their good and bad points:
KRPro has lots of settings, lots of downloadable tracks and supports VR. However, graphics are so-so and there is no AI to race against. The UI is a bit clunky too.
KK is still technically in early release. It has fewer available tracks (at least at the moment), but supports VR and has AI. Graphics are better than KRPro as well. But it's UI is "unique" and I feel that the Karts are a bit jumpy due to a very small dead zone that I can't get out of my wheel.
Kartsim for rF2 is a paid add-on that's fully integrated into rF2. That means that if you like rF2 (and Karts), you'll like Kartsim. The karts are available in the regular car list so can run on ANY rF2 track. VR is supported as well. The UI for rF2, of course is not everyone's cup of tea.
When it comes to driveability, I'd rate KRPro and KartSim for rF2 over KK at the moment. Just my opinion though.
FWIW - another option for karts (at least super karts - I can't recall whether "regular" karts are included) - is Automobilista - a great overall sim. The super karts are lots of fun IMO.
I hope this helps you a bit with your decision, and doesn't add to confusion.