I hadn't seen MPD before (well, unless it's a fork of ye olde mserv, but it doesn't look like it is). Off the top of my head:

• Chorale knows when it has MP3 and FLAC versions of the same file, so you can make a playlist once (in Rio Music Manager, say) and it plays back in MP3s on a MP3 player and in FLACs on a MP3+FLAC player;
• MPD supports Vorbis and AAC;
• Chorale's network protocols are the standard UPnP ones, so you can use third-party hardware (eg Roku Soundbridge) with it, as opposed to MPD's custom protocol;
• MPD does gapless and cross-fading (have you seen the amount of code you need to do that in Gstreamer? one day Chorale will throw out Gstreamer and do it properly);
• Chorale is a big cross-bar of music servers, music outputs, and music controllers; MPD is one-server, one-output, one-controller, and all the music must reside (or be mounted from) the server with the audio output in;
• MPD supports Internet radio but not Freeview; Chorale supports Freeview but not (yet) Internet radio.

Really I'd say MPD isn't an attempt to solve the same problems...

Peter