There was actually a scientific study done and there's not a nutritional advantage to feeding one over the other. However, adults are more nutritious than pinkies, which are mostly fat, and the nuritional value (protein and calcium, mostly) improves gradually as they go through their life cycle. Pinkies are advantageous if you're trying to fatten up a bp for breeding, but you don't want to feed just rat pinkies because of the lack of nutrition. The advantage to feeding rats over mice is that some bp's are reluctant to take more than one prey item and you can feed a full meal in one shot for those. Other than that, you can keep feeding mice forever, as long as the bp is getting enough to eat.