I run both sizes:
- http://www.4crawler.com/4x4/BFG_AT_KO.shtml

In fact now I am running my two old spare 9.50 MTs up front and two met 10.50 MTs in the back, makes a nice combination. I think the 9.50s are only available in the AT tread now, I think the 10.50s work a bit better off-road on 6" rims than the 9.50s do.