Should i get a rat or a guinea pig/ cavy?

I want a pet. I have decided either a guinea pig or a rat but i don't no what would be best. I am in 2nd year I go horse riding but can't run a lot because my knees will dislocate. I can keep them in my room or (preferably)outside/ in a shed. I could spend up to an hour to play with them but they could get out in a run, garden, room etc depending on what it needs I could clean their cage, hutch etc 1-2 times a week could pay up 2 �100 for their cage/ hutch. and if you answer could you not just say guinea pig or rat as it is a time waster but explain why and maby give a little information about them. thanks