Josephine,
Mice and rats are both great pets but like Breyer said, you MUST have at least two rats if you decide to go with rats. If you cannot devote a lot of time (and money, for things like small animal veterinary care) spacious housing, and attention to all the various details of learning how to properly care for rodent pets of any type, then, that is something you need to think about.
The worst thing a person can do is to decide off the top of their head that they want a certain type of pet, without thinking it through, or researching that type of pet thoroughly and considering all the pros and cons, or whether you have a living situation that may change suddenly, a roommate or spouse who might be allergic, landlord policies, etc, and after impulse buying, several months down the road, you end up having to give up your pet to a shelter, etc, and the pet ends up getting the short end of the stick because the person did not plan or learn enough before running off to get a new pet on impulse.
So, I would research, read and learn TONS about mice, and rats, and make a decision on it after you have learned more about them, so you can make a good choice for yourself and especially, for the welfare of the pet and their happiness.