I get my balls on rats asap. I don't know if one has more nutrition over the other but I do know that my med and larger balls could suck down mice for days and with rats it only takes one med or large and it cost much less. Also because the rat is a larger bulk of meat it takes a little longer to break down and seems to be retained a little better then mice.
As stated in a previous post for the size of the rodent look at the thickest point on your ball then look at the rodent and picture it laid out not all curled up in a ball eating on something or what not. When the snake eats it should have a modest lump but should not look like it just ate of foot ball.
Hope this helps,
matt