Studies have shown that there isn't much difference between mice and rats as far as which is better. The bigger difference is the age of the prey - older have more calcium. Therefore, an adult mouse is better than a rat pup. That said, I've heard of some breeders rotating between rat pups and adult mice for their young growing snakes to ensure they get all the stuff they need. Feeding more smaller items in one meal is better than a single large item.
I feed my 11 month old BP 2-3 adult mice per week. Sometimes he'll take 3 and other times just 2. I'm sure 1 single large item would be more convenient for me, but I'm concerned more about him. It's easier on digestion and I think the 'hunt' may serve as some form of enrichment and stimulation for his senses.
Just my $.02. 
-----
Check Out My Albums
1.0 Ball Python "Aragorn"
1.0 Kenyan Sand Boa "Gimli"
1.0 Saharan Sand Boa "Frodo"
0.1 Rough-Scale Sand Boa "Arwen"
0.1 California Kingsnake "Gentoo"
1.0 Mexican Black Kingsnake "Indigo"
1.0 Snow Corn snake "Chile"
0.1 Australian Cattle Dog/Pointer
"Kira"