My girlfriend and I have a tiny toy chihuahua. I got it when it was six weeks old. now it is about 5 months old or so. I trained it to pee and poop on pet diapers (mats)(the things you lay on the ground so they can use the restroom...forgot what they are called.) I have an 8 to 5 job, she stays in her room all day and uses the restroom no where else but on the pet mat. She is a great lil dog. She takes care of her self all day, we just leave food out for her and she'll eat throughout the day.
I am not a big fan of cats. I dont like cat hair all over the place, especially on my furniture, clothes and etc. but they are good pets, they do take well care of themselves .....and you dont have to shower them, vs a dog, you do. (There's another plus)
As far as shots, they (cat, dog) have to get about 4 to 5shots throughout the first year. Every visit is about 45 dollars and plus the shots. Then, there is the rabi shot, which is a little more expensive. Then if you dont want them to breed, thats another expense. (all this is what we have to go through with my lil chihuahua).
If you have time to train a dog, I'd say get a dog if not a cat or a lizzard or something.
You should get a parrot, that way, you have some to talk to when you are up late studying....just a thought.