Well, it depends.

Guinea Pigs:
For a guinea pig you have to be prepared to care for them for over 7 years. Their needs are pretty simple, fresh veggies, pellets, hay, etc. But they do need to be housed in groups on two or more. They shouldn't be housed with rabbits or any other small animal besides their own kind. They are very sweet natured, my guinea pig loves to be cuddled and just sit in my lap while I watch tv. Try checking out cavyspirit.com for more information.

Cockatiels:
I don't know a lot about cockatiels, so you might want to wait for some of the experts to answer your thread! But, I'll try!

If you get a cockatiel look for a hand raised one. You have to be prepared to take care of it for 20 years or more. They have specific nutrition needs and need just as much care as a guinea pig; they both need to be out of the cage every day, regular vet check ups (witha guinea pig, an exotic animal vet and with a cockatiel an avian vet.)

Hope I could help! If you have any guinea pig questions feel free to PM me!