I don't have any personal experience using Radeon graphics with a Linux-based installation of XBMC. (I run XBMCbuntu on a little Revo with Nvidia ION.) However, I've repeatedly seen posts mentioning that Radeon/Linux support isn't as well developed as Nvidia/Linux support. I'm sure a little searching around the forum could provide you with more detail
In my experience, however, any Linux install brings with it a one-time frustration session (which I've dubbed "the Linux dick-around") as you get everything configured properly; so maybe the Nvidia or Radeon graphics will just bring different elements to that dick-around, and following that setup, either can work well.