This video compares the boot speeds of Raspbmc and OpenELEC. Raspbmc June update and OpenELEC 4.0.7 were used for this comparison and both have XBMC 13.1 Gotham. The same MySQL video library was used on both OSes. Again the comparison is only in terms of boot speeds and nothing beyond that.
In my previous comparison on my blog (link below), OpenELEC was the clear leader. However, Raspbmc has caught up surprisingly fast. As you can see from the video, Raspbmc boots in about 1 min while OpenELEC was up in 30 sec. Other than the slow boot speed Raspbmc was neck and neck with OpenELEC in browsing through libraries and playing videos.
http://www.htpcbeginner.com/xbian-vs-...
The question is how often are you going to boot or reboot your Raspberry Pi. Mine is left on all the time. So this takes out the slow boot in Raspbmc. OpenELEC's main purpose is a simple stable media center. Nothing more. Raspbmc on the other hand has almost a full OS as the backbone that allows expansion. It also comes default with SABnzbd, FTP Server, Browser, etc. So if you ignore the boot speed, most enthusiasts who are comfortable on a Linux environment may actually prefer Raspbmc over OpenELEC.
The only issue I have noticed with Raspbmc is that sometimes when rebooted it freezes at the "Relax, XBMC will restart shortly" message.