In my opinion:
If one wants a Raspberry Pi for FRC, it should run Angstrom Linux. If it can't, one shouldn't get a Raspberry Pi. Perhaps think about getting a BeagleBone Black instead.
Why?
The 2015 control system will have Angstrom Linux as its O/S. Getting used to the abilities and limitations of a target platform early will get you up and running early in 2015 while still providing a very usable platform for 2014. Special thanks to the Robobees, 836, for sharing this info in their 2015 Alpha Testing session this past weekend.
http://www.ni.com/linux/ for some helpful info.
Beyond FRC, either is a fine choice. Though for robotics, the expansion boards can up the cost of a Raspberry Pi up to match a BBB for equivalent functionality.