I do not have experience with either in a ukulele, but I have K&K's in most of my guitars. They are passive, and to get best sound use an external preamp like an LR Bags PADI. I do sound for a local folk and work music concert series with national and international musicians. I pay quite a bit of attention to the pickup systems people use. In guitars, I have never heard a better system than a K&K. I have heard equal, but none better. Moreover the K&K is dirt cheap. That's why I am putting them in all my guitars and replacing the B-Bands, and other pickups. I really prefer a passive pickup so that I don't have to worry about a battery crapping out on stage too.
My Pono came with a pickup, however if my next ukulele does not come with one, I plan on trying a K&K. Will report back when this happens.