Very hard to beat a KoAloha soprano. They sound huge for their size. I played both back to back a few days ago and I highly recommend the KoAloha. Kiwaya is a quality uke as well.
Such different sounding instruments. I've owned a KoAloha soprano and a KTS4 & KTS7.
If you like a very loud and bright, more of an 'island sound' ukulele go for the KoAloha. They are amazing - really easy to play and loads of projection.
If you want that old timey Martin sound go for the Kiwaya.
I'd say the Kiwayas are the best built and personally I think Kiwaya make the best factory built sopranos that you can find.
I have a Kiwaya solid mahogany, the KTS-4; it's beautiful, and it's faultless. But I've never played a Koaloha or heard one live..... I understand that the sound is just completely and totally different. If I were, you, I'd choose based on the sort of sound you're looking for. I can't imagine that you'd be disappointed with either uke!