Get both, both well worth having- in my opinion!
Ferrets wont stop the rats comming.
I keep ferrets, 30 odd hens, dogs, etc..
My ferrets are always kept in secure spacious hutches. I have had one get out and do a bit of damage to the poultry once, but I forgot to put the latch on the hutch live and learn!
Best way to do it, like Skycat said, keep things like sheds hutches raised off the ground, and keep feed stuff well secured, in a rat proof place. As soon as you notice any rat shit, diggings, eggs being nicked etc.. get 'em sorted straight away. Even better, set up some permanent tunnel