Like a few of the posters before me, I had a ferret and he was awesome. He would even got to the bathroom on command so you didnt have to worry about him shitting everywhere. The average lifespan on them is 7-9, but they become more susceptible to disease at 2-3 years which often ends their lifes early. They make great companions because they have so much personality and might be nice if you dont want quite as long a commitment as you would get with a dog.
However if you do decide to get a dog, I strongly suggest that you get one from your local shelter. They are taking in way too many pets these days and need all the help they can get, and the best dogs I've ever owned have all been mixed breeds, because often times you get the more desired qualities of each breed without all the temperment and health issues.