Ferrets sleep most of the time. It's the time they are awake that they are running all over the place. Ferrets don't particularly like to be held or cuddled--they are so active and inquisitive they are always on the move. Ferrets have a short life span compared to guinea pigs--5 years is the average. Ferrets are carnivores so their poop is pretty smelly and the litter box must be changed often. Ferrets almost always develop health problems--adrenal disease and tumors, lymphoma, etc.
Rats don't take up as much space as ferrets, but their life span is shorter. Ours lived around 3 years on average. They love human interaction if you handle them a lot, but if you don't they aren't so happy about being held. They are extremely messy. They don't use a litter box. They are prone to respiratory infections and tumors (especially mammary tumors, and they have a lot of nipples so possibly multiple tumors).
Just like guinea pigs, there are down sides to any pet that you are considering. Research carefully.