Be careful with cats, a ot of people are alergic and you may rune your odds on the dating scene (if this is an issue).
I vote for a puppy as you can train them to be a devoted companion... And, training is important. If you have 2 other roomates, maybe they could get involved in potty and obediant training too. I recomend kennel training while you are away and they like their own little den.
We work a lot and spend the entire weekend with the dog. We work from 7 - 6 and I think "Moose" is happy to see us leave on Monday morning so he can catch up on his sleep!
Dogs rule!