I'm gonna have to go with shrek kart here. Krazy kart is fun, but a lot of the levels look the same. Props are reused over and over again, making it somewhat dull.
They are both amazing, but Krazy Kart. Shrek: better graphics, about the same content, less sense of speed, interesting power-ups, unlockable tracks and characters Krazy Kart: blocky, but nice graphics, good sense of speed, Konami characters (Robbie the Rabbit, the frog in Frogger, etc.), unlockable tracks and characters, DLC (but nothing has been added to it yet) Basically, if you have enough money get both. If you only want to spend 0.99 then go with Krazy Kart but still keep Shrek Kart on your list, it is awesome. But Krazy Kart. I can't even make my decision and I'm not the one even buying them.
I think Krazy Kart has better handling, but Shrek Kart is better in overall design. They're both $.99 at the moment, just get em both, try not to pay more for them I paid $8 for Krazy Kart and for that price it was not worth it. For a buck you cant complain so get it before the sale ends.
