I just finished doing a comparison of Xbian 0.6 and Raspbmc RC4 (for science!).
Samsung 37" 1080p TV, pi connected through HDMI.
1TB External drive (powered) connected to the pi.
I used the same SD card, and the default settings for both, and tried out a few different file types. My basic conclusion is that there's little difference performance wise
CEC worked perfectly out of the box on both, and the menus were very smooth in both. I also used the iOS official XBMC app to control both, and it worked without any problems.
First - MP4 (2.0 SD AAC, the newer format for scene release TV shows). This played back perfectly in both.
AVI MP3 2.0 SD - played back perfectly on both.
AVI Dolby 2.0 SD - played back perfectly on both.
720p 5.1 DTS - played back fine on both (there was some initial occasional HDMI dropouts but this was sorted by changing the XBMC settings, I cant remember exactly which one though).
1080p 5.1 DTS - frequent stutters on both, and heavily distorted display (artifacts in the picture all the time).
I tried a few addons, all of which worked perfectly - iplayer, itv player, 4od, demand 5, tvcatchup.
I tried playing about with the settings for the 1080p file but couldn't get it to play back properly in either distribution. I even tried another file just to rule that out, but had no luck. I'm not that experienced with XBMC so I didn't really know what I was doing (I am experienced with video/audio files, but not XBMC specifically).
One thing I've read is that changing the resolution to 24hz helped a lot of people. I tried changing it and got a confirmation popup. I selected yes, but as soon as I exited the menu it reverted back to 60hz - this happened in both distributions and I couldn't get it to stick at all. I tried tweaking things like 'sync refresh rate with video file' etc.. but had no luck. I don't think its related to DTS because my TV can handle it so I assume its just being passed through.
So... overall, I had a pretty much identical experience with both. The one thing raspbmc has going for it right now is the automatic updates (but I assume Xbian will get that eventually). Xbian is updated far more frequently (the downside is having to re-image the SD every time).
I was planning to try openelec as well, but realistically I'm more likely to stick with one of these 2, so I left it for now.
If anyone can help with my 1080p or resolution issues, then I'd really appreciate it