Why Gerbils are the Best of all Cage Pets?
Reptiles and fish -- they are pretty cool to watch, but more of an "accessory" than a companion.  We have a leopard gecko which is the cleanest, most low-maintance creature you could imagine.  It is fun to watch her hunt crickets and shed (and eat) her skin. But I am nothing more than a giant cricket dispensor to her.

Birds -- birds are loud and messy.  I had a parakeet when I was 13 who loved to sit on my shoulder and whistle back and forth with me.  She hated her cage and left bird droppings everywhere.  Birds demand a lot of attention, have smelly cages, and scatter their seeds.

Rats -- rats are as intelligent as a dog; in fact they remind me of a puppy-chimp mix.  This is big commitment to keep their mind and need for companionship occupied.  They have a large housing setup that is time consuming to clean out.  The fur/cage has a "rattie" smell to it.  Our two rats use a litter box, but I hear that is unusual.  Unfortuately, most people will not understand why you have or love your rat.

Ferrets -- ferrets are expensive to buy and house, and a big time and energy committment.  Moderately smelly, but use a kitty box if you don't mind dealing with those.

Chinchilla -- chichillas are sort of like giant, very soft gerbils.  They are not still for long. They have fragile bones and insist on gentle handling.  Females are more aggressive and more likely to nip strange humans.  They are a long-time committment (live about 10 years).  They are hard to breed and have only one baby after a four month gestation.  A nice pet for adult/experienced keeper, but an expensive pet who should live in pairs (so about $300 for the pets and housing).

Mice -- not much to them.  They are not truly "cage pets" and have escape on their minds.  It's more like having a miniature prisoner than a pet.  My sister had a mouse when we were kids.  It loved to use its superior brain to escape out of our latest "escape-proof" housing, go through the heating pipes, and terrorize our rodent-phobic neighbor.  For such a small creature, they have a powerful smell.

Bunnies -- bunnies hate me.  Every time I pick up a bunny I end up with a set of giant claw marks on my hand and wrist.  People claim they can be house broken, but unlike ferrets, this can be tricky to acheive.  Bunnies are not actually cage pets and need to get out of the cage and hop around every day.  This means you need a large bunny-proof area for them.  Bunny-rescue tells me they should be spayed or neutered to make good pets.

Guinea Pigs
-- another nice pet.  Guinea pigs can live in groups and are gentle, people oriented animals, that are nice to hold in your lap.  The only reason gerbils are better is that guinea pigs have more expensive housing requirements, they are not as curious or althetic as gerbils, and you have a 10 year committment on your hands.

Hamsters -- gerbil people are very jealous of hamsters.  Hamsters have the popularity that rightfully belongs to gerbils.  While hamsters have cuter faces and softer fur, they are just not as nice or fun as gerbils.  They are nocturnal so usaully all you see is the back of your hamster sleeping in a ball in one corner.  They are not ready to play until it's time for you to go to bed.  Then they obsessively run that wheel, th-th-tha-thumping all night long.  Hamsters smell up the cage very quickly and will pee on you.  Hamsters are solitary animals, so you need to keep one per cage.  This makes keeping them more expensive and breeding them more difficult than gerbils.

While I wouldn't take it that far, there are those who subscribe to the theory hat hamsters are evil

Gerbils -- gerbils are great. They are dinural, so are awake night and day.  They get very excited about any new thing you put into the tank for them and will immediately explore it.  You can make cardboard buildings/tunnels for them and they will love crawling into, through, over, and on them.  Then they will gnaw it all up.  You can house two for a $25 10 gallon set up.  They are by far the cleanest of all the small pets, so bedding is not a big expense.  They live in clans and groom, nest, play together.   You can house up to seven boys in one tank (five gallons of space per gerbil).  They are easy to breed and both mother and father are great parents.  Gerbils come in about 40 colors and varieties.  They are not a big commitment as the life span is about  3 1/2 years.  I have twenty something gerbils.  One tank of four colorful boys is in the family room on top of the television.  They are more interesting than the majority of shows, so I'll sit with the family and watch "Gerbils TV" instead.  Cats also love gerbil TV.