Personally I'd go with the weans. You can always feed two weanlings to get the right total prey volume as he grows but you can't make a small rat smaller if they are too big right now for him. You could split the order and get both but I'm not sure how many snakes you are feeding or what sizes they are and the amount of rats you are ordering.

Also don't hesitate to actually phone and ask for Edna. She's co-owner of that company and a really helpful, nice lady.