I've heard that rats are supposedly more nutritious than mice....I've never heard that mice have more calcium.

Eventually, you'll want to be feeding your bp rats because they're bigger. You can feed one proper sized rat...or a handful of mice. Just makes more sense to feed one item at a time, if you can. Many bp owners will move their snakes onto rats as soon as possible. But some stick with mice. It's really up to you...and your snake, which can often be finicky beyond our understanding.

Does any one know why BP's sometimes will only eat mice in the winter.
I don't know. It's not uncommon for bp's to go completely off food in the winter. Maybe some choose a smaller meal for similar reasons that some choose to fast completely. Could be that seasonally cooler temps make digestion potentially more difficult, so they will either not eat, or eat something smaller than normal.