I have played many of both brands and love both. I worked for a long time to get dealership with Collings Guitars almost eight years ago. They're my favorite factory guitars. Had a C10 for a few years and am a big fan to say the least. Bill Collings is a genius in my book. He was a machinist and that's the stand point he approaches it. I sprayed all the laquer for Ko'olau for many years and I can truly appreciate the level fit and finish that Collings comes with. Even Bob Taylor said that Bill was the best guitar maker. His mandolins, archtops, electrics, ... phenomenal!
You know what though, Both Collings, and Kamaka are very consistent, IMHO of course. A lot of commentors would argue that, but both extensively use computer aided technology for thickness, brace shapes, etc to be exact and I hear consistency. The Collings is built lighter, while the Kamaka is a bit more solid. I owned a UT-1 for a bit, but it started getting a belly and the neck even started going beyond what you can call relief. This was after 6 months with Koolau Golds and somewhat humid weather. Collings, like a number of builder's, go for resonance and many base their calculations on the tension of Aquila strings or the lighter Worth strings. If you don't prefer those strings, you have to consider the nature of what you have and what it can handle. Martin will void the warranty on your guitar if you use heavy gauge strings and on certain models even medium gauge. . The point is to be aware. Collings and a number of other makers out there are building for ultimate tone right off the bat and it can be absolutely glorious, it's undeniable. But those in the market need to know that their instruments may have been built to handle a certain tension. Or, at least that's what I've experienced. Ask the manufacturer.
Kamaka's typically will have slightly less volume than a Collings, but have excellent balance and a sweet voice. Ya, they have the open pore gloss finish that no guitar maker could get away with, but with Hawaiian uke makers this has been the traditional look for the last 100 years. I'm not the snob I was in my younger years about such things. Every time I pick up a Kamaka of any size I just want to jam, take it home even! They're excellent ukes to play! And...I can put golds and most other high tension strings on any Kamaka model for it's entire lifetime and it will handle. I can't speak for all Collings, but mine was not diggin my string choice. So ya, just be aware of these things. At the end of the day I'll have fun with whatever I grab. my kids u900, Just a beer and a little laminate uke made in china, woot woot!