No, there's really not that much difference. From what I've heard, bloods are normally not as picky as bps and seem less likely to "imprint" on a certain type of prey, but I suppose it could happen. Since an adult mouse is bigger than a rat pup and technically has more substance for your snake at the moment, you could either switch to one adult mouse, or just go with two rat pups instead. It's really up to you, but since you said rat pups are hard to find in your area, mice might be easier for the time being.