I'm going to immediately break your request, being a person who's only used a kaoss pad 3, to say that the two choices are completely different options, and it really depends on what you specifically want. It's an apples to oranges thing, really.
The kaoss pad (3, at least. haven't used the quad, but it's a completely different device. simultaneous fx, but no sampling/looping, and no über-cool led-grid pad.

) is definitely not the greatest fx processor out there, but what it lacks in quality it makes up for in its versatility and the number of fx options it has. It's really not as bad as some people say it is, but I wouldn't, say, run a master mix through it.
It's also loads of fun to use.
The sp serves a completely different purpose. Someone else can comment on its value, but it's still ultimately up to you, what functionalities and possibilities you want to have.