Koa's the trad wood for Hawaiian ukes. The big 'K' makers seem to specialise in koa instruments.
At the end of the day, all that matters is what you like. Personally I prefer mahogany. It sounds warmer to me - a little less brittle. That said, koa seems to deliver volume.
Doubt I could afford any of the koa ukes I've tried.
Actually, Kala make a line of solid acacia (related to koa) which are really nice. A different sound... Sort of koa-ish, and fair volume. Worth a try.