I'm assuming you are soloing. If so, then it is purely preference. I have found that you can get more exp per hour doing smalls. The reason being that the dungeon isn't as spread out as a medium.
For example, I can finish a medium in roughly 30 minutes and a small in roughly 10 minutes. You can gain anywhere from 9-15k exp from a medium and 4-7k from a small. This makes the exp gain...
