I have owned a Guinea Pig, cats don't eat them if their in a cage or anything lol.
I personaly perfer cats, I have had many Guinea Pigs, yes they are ADORABLE,
But they die sooner than cats, and cats are more the pets that give you company.
Because Guinea Pigs generally stay outside.
So honestly, I recon you should go for another cat, or if you want to try something different, GP. :]
They are the cutest things, but wow.
I can't choose now xD!
Its really up to you!
~Mango