The jury is still out on the Mini, since it has not yet been released. The Nitrus, from both my personal experience and others here, is not the equal of the Zen. You may be happier with the Muvo2, but I have not listened to it yet so cannot say for sure. Nevertheless, you may find all these options are "good enough."
I have yet to hear a flash-based player that is even remotely the equal of the Zen. However, I was very surprised at the Muvo's sound quality; it doesn't have the dynamics of the Zen, nor the very deep bass, but it sounds relatively smooth overall and generally pleasant.
BTW, if you can wait for the Mini to be released, you may yet find it for under $200.