Based on my experience, the Pono.
I have not tried both of these exact models, but the Ponos I have played (especially the one I own) are very high quality and play well and sound great. The Islanders I have played are nice enough, but I have not seen one that I thought was all that special. There is a local musician that we see regularly who recently got an Islander ukulele and has used it at a couple of gigs. I am not really a fan of the sound - It's hard to explain, it's just - plain I guess is how I would describe it.
I really like Joe Souza and Kanilea products, and I really want to like the Islanders, but I have just not been wowed by any of them yet. I find them to be similar to other import brands like Kala and Ohana. The Ponos I find to be typically better than those.
That is just my opinion, the best advice it to try them for yourself.