2 1/4" vs. 2 3/4"

My son is 11 years old and is not that big for his age. We are looking at a new bat and simply presumed that we would be getting a 2 1/4" bat with a -11oz or -12oz drop. We've been advised, however, that our new league is not adopting the little league standard and will allow big barrels. So, the question I have is: will my son be better off swinging a 12oz drop 2 1/4" (say a 29/17) bat or a bigger, but presumably slower 8 oz. drop 2 3/4" bat (say, a 29/21)? In other words, what's better for the young kids, speed or thickness? I am really confused and would appreciate any suggestions. Thank you all very much.