I've had both and prefer the guinea pig. They are cute, small and you can get a leash and walk them! I had a cute black curly haired guinea pig. They can be very messy but it's worth the effort.

My two hamsters were male and female and let's just say no one slept at night! Try explaining to a four and six yr old why one hamster is on top of the other and why the male is slinging the female from one side to another! Mama Mia! lol

We wore gloves to clean the hamster cage but they never bit at us until the babies kept coming and coming and coming and coming. If you get too many babies momma will eat them. That's another hard question to answer to a young child.