amarra or pure music..?

They both sound great. I agree with your assessment of the differences though. Unfortunately, PM is not yet stable as you also point out, which gets very frustrating for me. Hence, Amarra gets my vote.

The price of Amarra is what has kept me from trying it. PM sounds great, and the latest version runs pretty stable if I turn off the "update in remote" switch. Amarra 2 was just introduced.
I own PM, but am waiting to try Twilight when it gets out of beta.

I can't comment on pure music, but can tell you that the first time I've ever been able to say "that sounds really good" without adding the words "for digital" was with Amarra on my Mac Mini feeding my Ayre QB-9. Some of the differences are obvious, but many are subtle and require some extended listening to notice (or not notice the low levels of grunge and harshness that pass for normal with most digital). Since PM is relatively inexpensive and Amarra can be checked out in demo mode for free, why not try both? I've been meaning to do that comparison but haven't gotten around to it yet. (Or maybe I'm afraid that, since I own Amarra that I wasted a bunch of money). In any event, Amarra is a pretty tough act to follow