Whichever your snake will eat consistently works best.

As far as sizes - for BPs and other girthy snakes look for a rat with about the same girth as the thickest part of your snake. Terms referencing sizes can be awfully varied between what you might see on feeder sites and what the interpretation of that might be at your local supplier etc.

Hope that helps. Use the search feature and I bet you can find a tons of threads on rats vs mice.

searching string: rats mice
option: search titles only
result: http://www.ball-pythons.net/forums/s...searchid=48131