I have the Kala and I've really enjoyed it. I've tried the Pako at the Kanilea factory and it seemed nice as well, though I didn't have a chance to plug it in.
I definitely prefer the Pahoehoe strings that come standard on most models of U-bass. I like the fat rubbery feel. The Aquila Thundergut strings that come on the Islander have a sort of sticky feel that makes it hard to slide your fingers up and down the strings. Some people strongly prefer the Thunderguts though. You can definitely use the Thunderguts on the U-bass, as they were originally designed with that instrument in mind. I don't know if the Pahoehoe strings would fit on the Pako without modification of the nut and/or bridge.
If you want an absurd amount info about the U-bass, check out the talkbass.com forums. They have series of megathreads focusing on the U-bass, I think there were at least seven 50-page threads the last time I looked.