Well, as a not completely satisfied owner of: eSata Sheevaplug: currently NAS, no video, soft floating point. Raspberry Pi B/512: was a music server but the built-in audio is extremely crappy, unbalanced GPU/CPU power (damn you, DTS tracks), now sitting in a corner. Cubieboard: (current music server, stable, faster than RPi but no video hardware acceleration in linux). I just bought this in without much thinking. I h…
Isn't Raspberry PI's gpu acceleration supported on XBMC (with the right driver and/or dedicated distro?)
Also, even if HDMI audio works for you, XBMC interface is sluggish in the RPi. I'd check anything else for serious XBMC use.