I'd go with a cat, rats, or guinea pigs as well. A cat will be more expensive of course, as shots, spay/neuter, etc. add up. But I think a cat gives more back (sleeping with you, possibly mousing if needed, a little more company in a way). But rats and guinea pigs enjoy being handled and are trainable and interactive. I've only had female rats, as the males just look too "obvious" to me.
I say go to the local shelter, ask to visit with their longest resident cat and adopt it if you and the cat get along (dont be surprised if its a male black cat, they seem to be the most over looked)
my two cents--- go to the shelter and ask what cats ( yes my vote is for feline - although I am a true dog person - kittys are cool )
are in line to be put to sleep - take a older female cat one with a sweet face- kitty are a perfect pet for you
The thing you need to be thoughtful about is what you will do next time you move. WAAAY too many people get pets without contemplating how long the animals are likely to live and therefore how many years they will be *responsible for* continually finding somewhere to live that allows such pets.
In some regions it is not that hard to find affordable apartments that allow cats or small dogs, in which case i'd say an older cat used to living alone might be perfect for you; however in many regions of the US it can be extremely hard or even near-impossible to find a rental place that allows pets yet has an affordably low rent, and if you live in one of those sorts of areas (or have any realistic chance of moving to one within the next ten years or so) it is not fair to an animal to adopt one now.
If your future plans are somewhat up in the air, have you considered seeing if you can FOSTER a cat or dog? Either for your local shelter or for rescue groups. As long as you are willing to take care of it in the way that they want (e.g. if they want the cat to be indoors-only) that can be a GREAT way to have a pet without worrying about "what about when I move".