I have had many chinchillas and 5 guinea pigs. I would say guinea pigs make better pets than chinchillas. I would go as far as to say that they are very much equal in the majority of their needs.
Here are several things that make them similar:
1. Diet. They both require pellets, hay and fresh water. Additionally guinea pigs eat a variety of veggies daily, not a big deal.
2. cage size, chinchillas can jump 3 feet high. They need a very roomy cage, primarily vertical space. But if you let your chinchilla out for cage time they will run fast and far and rebound off the walls like a skateboarder on a ramp. Guinea pigs need horizontal space. Size wise the cages would be about the same, chinchilla cages would be a tall rectangle. Guinea pig cages a long rectangle.
3. They live in the same area of the country and have the same temperature requirements, both need to stay under 80* to avoid heat stroke, both prefer cooler environments.
4. they both can go on either disposable bedding or reusable fleece bedding.
5. they both poop ALOT. ALOT.
6. they both beg for food.
7. they are both equally hearty. As in there is no difference between chinchillas an guinea pigs as far as which is more or less hearty.
8. Neither should be housed outside.
9. they both shed fur.
10. they both have teeth that continue to grow.
11. They both have fully furred open eyed young, able to run around from the day they are born.
12. Vet bills are about equal.
13. They are both very fast when loose.
14. Both are herd animals and both benefit from living with a buddy. Anyone who has seen chinchillas groom and snuggle know they should be in pairs.
Here are the main differences:
1. chinchillas are nocturnal. They are kinda boring during the day. They are very active and can be noisy at night. Chinchilla wheels take up space and are expensive but provide a great outlet for exercise. If you are a light sleeper they will wake you up.
2. Chinchillas require dust baths. Even with a bath house (plastic box) this makes a mess, increases dust in your house and is terrible for asthmatics and those that suffer from allergies.
3. they live a very long life. Most of my chinchillas have lived over 12 years, everyone of them over 10 years. Several of them have lived to 13/14 yrs old. My current chinchilla is now 14.
4. Chinchillas do not like to cuddle. They will not typically sit or snooze on your lap. Guinea pigs are much more interactive.
5. Guinea pigs drink more often, they pee a lot more. Their cage requires more frequent cleaning.
6. Guinea pigs are more interactive vocally.
7. Guinea pigs can live to around 8 but the average life expectancy is 5 years.
8. Chinchillas can and will get into lots of nooks and crannies. They can jump out of most corrals. They really need an empty room to let them run in vs setting up a simple play pen. Once they learn how to jump you won't be able to keep them in it unless the walls are over 3 feet high. If you don't have a place to let them run then you might end up sitting in your bathroom a lot
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9. Chinchilla pellets cost more, but guinea pigs love veggies and those will cost more than the chinchilla feedl But chinchilla dust is an added expense so I'd say its about equal.
10. chinchillas are much more voracious chewers and more destructive if you leave them loose in your room they will probably find a way behind your furniture to any wires and chew them.
11. Chinchillas really need either a store bought cage, or a cage built with wire bigger than 1/2" x 1" wire size. You really shouldn't put a chinchilla in a C and C or any other cage with bars any bigger than that or when they bounce off the walls their feet will go thru and it can break a leg. This happens all the time and it has happened to one of mine.
12. Chinchillas are easier to catch, by putting down their dust bath and waiting for them to jump in. The trick is not to dust them before you let them out to run.
13. Poop frequently gets flung out of a chinchilla cage.
I love my chinchillas, but guinea pigs make better pets, especially if you are looking for an animal you can pet and cuddle.