I found this question in several web pages but it is not correct speaking about Raspbian vs Raspbmc because:
In fact Raspbmc runs actually on top of Raspbian. The main difference is between their GUI (graphical user interface). In fact while Raspbian comes with a lightweight desktop environment , Raspbmc does not. Raspbmc has one Graphical Interface (the one from XBMC) which cannot be used for the usual desktop operations like copying/moving files and folders, edit texts and so.
On Raspbmc you cannot install programs like a web browser, even if a web browser for raspbmc capable of playing flv movies would be definitely great!
Another main difference of Raspbian vs Raspbmc is that Raspbmc is built stripping down functionality from Rapbian and consequently it is faster, having to take care just about the few programs it consists of.
If you want a very small, portable, HD video capable media center then you should go with Raspbmc. If instead, you need a web browser, a development environment, an email client or a word processor, then you definitely need Raspbian.
Install both on the same sd card.
Anyway, since Raspbmc has been built on a stripped Raspbian, you can install XBMC on Raspbian (article in italian).
Consider using 2 sd cards
I love to discover way to use the Raspberry-pi (even flying it on a quadcopter sending telemetry data via wifi to my macbook), that’s why once i find a useful things wich can be done with one or another sd image, i use to keep them, and with at least 2 cards you can use onesd card to watch downloaded movies keeping the wife happy, while at night time you can do you evil things with your raspberry using the evil sdcard. ^^
Tell me what do you think with a comment.