There's multiple differences between KAR and SRi. It's not a copy, and even if it was (for god knows what reason) based on KAR , it kicked Air Ride out the window with an Egg Feet machine powered by all seven chaos emeralds.
Incomplete list of differences:
Pro KAR
-Ability system allows for multiple temporary styles of play (kirby only)
-Excellent multiplayer modes
-Enemies in stage might give you a fun level of higher difficulty
Pro-Sonic Riders
-Trick system implemented in SRi
-Based on multiple buttons instead of the "simple stick and button" style; moves and actions are seperate from one another and more easy to control
-Many, many more styles of playing (i.e. use a different character-board combo)
-Many shortcuts, and different levels of accessibility for them
-Different characters have different stats; not simply different vehicles/boards
-Level-up system
-Fuel system, allows for non-stop-charge boosts (press the B button and you can sacrafice some of your air for a boost)
-Story and storyline mode, even though we all know Sonic stories aren't perfect anyway
-Different characters aren't forced to use ___
-Much easier to attack opponents melee-style, and can control when you want to
Strange and peculiar similarity between the two:
-Both have unlockable obese characters on extreme bikes.
Anyway, even though Kirby Air Ride is a great game, Sonic Riders beats it. And they're not really copies if you look at it closely. So get Riders if you haven't already, and have fun with it.