The thing is that even both karts are no longer made, I still think you'll have better luck getting spares for the Kyosho. However, if you are crafty and can make your own parts, the Go 'N Play kart may be a better deal.
Also keep in mind that a 1/4 or 1/3 scale kart still isn't very big. Probably about the same size as the typical 1/10th scale ST, if I remember correctly.