I'm a happy owner of both a KoAloha and Kamaka tenor. Comparing the two is apples and oranges, really. It's hard to put it into words, but in a nutshell the KoAloha is LOUD LOUD LOUD,and it has a sort of throaty tone (sometimes I can imagine it sounds a little like a Dobro!). The Kamaka isn't as loud (and it's true, there's a big variation within a particular Kamaka model), but the sound is more balanced and "focused". Granted, when I bought my Kamaka I was able to play 3 of them and I picked the brightest and loudest (it was also the one with the least-figured grain, but that doesn't really matter to me).
If you're an acoustic guitar freak, I could give an analogy (heck, even if you're NOT a guitar freak, I'll say it anyway).
Compare a nice old Gibson J-45 to a Martin 000-18. The Gibson hits you with a real "gut punch" and begs to be played hard but rewards you with lots of volume. The Martin has more crispness, a more "refined" tone, but it can be easily overdriven. Well, the KoAloha would be the J45 and the Kamaka the 000. They both have their fan clubs, but it all depends on what you're looking for tone-wise.
Build quality: Both pretty good, but the "fit and finish" of the Kamaka is maybe a little bit nicer. At least they are comparing my 2005 KoAloha to the 2010 Kamaka. I don't know if KoAloha has improved on their finishing procedure, but in my case, they could have used more filler on the sapele neck before shooting the lacquer. Also, the KoAloha has a bound fingerboard with the frets ending INSIDE the binding (I don't know if they still do that). Not a fatal problem by any means, but it might take some getting used to. The Kamaka has an unbound fretboard with the frets extending all the way across the width.
Finally, there's after-purchase support. KoAloha really makes you feel like ohana when you have one of their instruments. Just this week I had an email exchange with a couple of random questions about design changes since '05 and I got email replies from the boss within 30 minutes! They really DO care about their customers. Also, their warranty will cover wood movement...something Kamaka wouldn't do.
My suggestion: bite the bullet and get both. Then you could keep one in Low-G.