This is a question that is often wondered, but seldom asked. Often times, prospective owners choose their pets on emotion or happenstance when a pet jumps into their arms. This is a wonderful feeling and very hard to put aside, but you should always know what you're looking for because if you don't then you might find a mismatch that is more problematic than it could have been.
Choosing the right pet can be difficult, but it will always be successful if you take a little time to choose the right pet for you. Do you see yourself as a cat or a dog person? Do you see yourself playing with your pet at the beach, or lounging with your pet around the house? Do you see your pet being a trip-buddy or one that stays home to hold down the fort while you're away? These are the first questions we should ask ourselves.
Say for example you want a very low maintenance pet who won't follow you around all day. In this case a cat might be your best option. If on the other hand you want a buddy to tag along on your adventures, then a dog would be best. Of course not all dogs are created equal and because of that you should try to realistically figure out how much time you can truly spend with your pet and then find the pet that does better in that situation.
When I picked up my first pet, a lab named Caleb, I had wonderful visions of the two of us throwing a ball around, going to the beach and just hanging out and enjoying life. I didn't quite realize that a Lab required a lot of exercise or he'd get into mischief at home, like tearing up some vintage albums, or a few $200 blinds, or a some pickets off my backyard fence or the garbage under the sink because that's where his treats were. Not that any of this happened of course because my Caleb was perfect (Right!).
The point is that I needed to adjust my life to accommodate my best buddy. That basically meant my life would change from one where I rode around the City on my bike for three hour rides, to one where I was spending an hour a day burning his energy off at a dog park. Not a bad swap, but one that I should have known about beforehand. Not that I would've gotten a different dog, because he was the perfect fit for me, but it would have set my expectation level appropriately.
For cats you can find good information here:
For Dogs, you can find great information here:
Good luck and I hope you enjoy your pet!
Jesse