These are the pick of the three karts we've tried.
Arrow IAME X30 Super (single gear)
Shifter Kart 125ccSuperkart 250ccMy take on them is that the Arrow is the hardest to drive, quite sensitive steering and can be snappy, but feels good when you get into the rhythm of it. (PSI: 12/12)
The Superkart is "easier" to drive, very fast, quick reactions needed. Circuits would be more open and flowing to allow for the greater speed. (PSI: 20/24)
Shifter Kart is "middlish" - easier to drive, easier to control, feels very natural to me, more kart-like. (PSI: 12/12)
All three are up on the practice server if you want to give them a go. I would reduce caster to around 10 to make the steering feel less sensitive.