On a sidenote, to answer the actual question...
IMHO Freshly killed is the best option for snakes, though f/t is certainly sufficient. Generally the parasites rodents are infected with are not the same kind that can infect snakes, so that isn't a serious concern. Freezing does break down cells and deplete *some* nutrients, so freshly killed offers the best of both worlds. Safe for the snake, humane for the rat, and 100% fresh nutrition