well i'm a big IPB fan, always will be (don't agree with their new policies tho) but saying that, i don't always use X-IPB forum and i can't really say all that much with regard to what sways me to using x-ipb on 1 site, and CBB on another :S
ipb has many functions though, CBB doesn't have as many, but it has the most essential functions.. i agree tho, if the admin area of it was more categorised it would be even more superb.
feature wise.. i'm going on my own usage and from other sites i manage that use ipb forum, the common features used, some which CBB has already..
admin side:
Moderator & admin Logs (the ability to see a detailed list of activities by each admin/moderator, ie. what operation they actually performed with date/time)
create password protected forums.
banners for each category?? not sure if cbb has this.
ability to set different background colours for each group, by this i mean if an admin posts then all their block is in a defined colour (ie. as it is now, .row1, row2 etc) but normal posts the same, example
http://chat.sundissential.com/index.php?showtopic=5379 ) this isn't a standard feature of ipb, but i did it as a hack for ipb on that site.
user side: members online today list.
more optimization on sql and http requests (there still seems to be a large pause when viewing the forum and lots of http requests before a page is viewed) not a serious issue, but it does make the forum far slower than x-ipb.
i can't really think of anymore to be honest.. that's mainly my opinion.. but speed is the main thing that governs my choice, not the features. CBB has come a hell of a long way though in the last year or so and it definitely gets better all the time..