A mouse. When you think about it what would you feed a rat? They are mostly scavengers(eat whatever they find) while mice on the other hand like eating food with seeds and things. I can also honestly say that I have never heard of anyone having a rat as a pet. Mice are cuter too and don't stink as bad as rats. Mice are actually pretty clean. My friend had one and it would only 'go' in a specific part of it's cage and would make itself a nest. the cage was organized and easy to clean that way.
Either,I have rodents too(gerbils) so in not sure but I think both would be cute and funny to watch.If your still not sure then go to google.com and type in rat facts or mouse facts then you will learn more about both and can make it easier.P.S. a lot of people think rodents are dirty but they are very clean.Beware of how fast they multiply though;) ~Cherry~