I've played both and posted about them on an earlier thread. As Kekani notes, the workmanship on the Collings was incredibly good; I looked at it for a long time and couldn't find any sign that it was made by human beings. I really liked the way it sounded, even though, as Kekani also notes, I was imaging it with Worths rather than Aquilas. It doesn't sound anything like a K brand, however. It's a more traditional mainland sound than a traditional Hawaiian sound. I've often heard them compared to Martins, but I didn't get that either, apart from the appearance. To my ears, Martins have more "ping" at the high end, whereas the Collings are more even toned throughout.
I have to say that I was less impressed with the Kiwaya concert. I had believed that it had Fremont Blacklines on it, since Kiwaya appears to use Fremont cases. Later, I read that they are not Fremont strings, but another type (just something I read but cannot confirm). So, the strings may have had something to do with the lack of robust tone I was expecting. Still, it was a fine instrument, and very well made.
I played about a dozen high-end instruments that day, so my comments should be considered in that light. Another way of stating the same thing is to say that I would be thrilled to have either the Kiwaya or the Collings... or both! I was surprised by the Scott Wise soprano I played. It sounded wonderful to me. The only thing that was a bit of a disappointment about it was the matte finish. Not that the finish was poor (it was very nicely done), but I could see that the koa was wonderfully flamed and a glossier finish would have set that off better, although it may also have altered the sound. The only new instrument that I was hoping to play was a Moore Bettah (none in stock). Good for Chuck, not so good for me.