Not all hamsters are sweeter than rats, and not all rats are sweeter than hamsters.
It's cost around the same, as well as time, to take care of them, anyhow.

Rats live around 2-4(?) years.
Goldfish can live for a LONG time as well, if well cared for.
A few fish would be well, if you buy a tank that would be secure from the kitties. My aunts cat used to eat her fish. It takes alot of work to set up a safe aquarium (water testing and so on) and one the fish would enjoy. Also, aquarium decor (hide away rocks, rocks to swim through, etc) can get rather expensive.


I personally like rats better than hamsters because they're larger, smarter (No offense, but it's true), and like Ash said they're like tiny dogs.