Out of mice and gerbils,I prefer mice personally. Gerbils are very high strung and hyper and my one gerbil was more destructive then my rats,mice and hamsters put together!
If you get mice,get a pair of females. They need to live in pairs or groups. A male mouse has to live alone,but unless you are prepared for a lot of smell (male mice smell bad),then it is better to get females.
Gerbils also need to be in same sex pairs.
You can have two female mice in a 15 gallon tank. I know someone who has two mice in a 10 gallon,but this is pushing it. I have two females in a 33 gallon tank right now. And a male in a 29 gallon. With mice you have to make sure they cannot escape from their cage and with hamster cages,mice can often squeeze through the bars.
Pros of mice.
+ they are small and don't take up much room.
+ easy to care for.
+ friendly and are tamed quite easily.
Cons.
+ can be stinky,especially males.With proper cleaning a pair of females should not smell.
+ when they get sick,they go downhill fast. It is hard to find a good vet that will care for mice.
+ escape artists.
Whatever you choose,make sure you do NOT use pine or cedar shavings. These are toxic.
.(NO RATS parents will not allow me to have any due to bad experience)!
