The raspberry pi's cpu is armv6 and a single one of the quad cores in the odroid would outperform it even when it's throttled to the same clock freq I expect. These devices will give you a bit more flexibility, for example if you want to do oversampling and digital filters on the computer.
These boards do look quite nice. There are a few arm boards available now and the most important thing now appears to be OS support for the board as the current linux kernel's need to be compiled for each processor arch separately.
I have archlinuxarm running on my beaglebone but haven't yet got to trying mpd on it. It appears that archlinuxarm does support the odroid-x
For anyone in the market for a board for these things I think it's worth keeping an eye on this - BeagleBoard.org - unzipped - revision of the beaglebone with onboard flash storage to run your os, allowing you to use the sdcard for storing your media files if you don't have storage shared on network.
Cheaper price hasn't been announced yet either though it is supposed to be announced by the end of this month.