The number represents the diameter of the bulb, in eigths of an inch. For example, a T5 bulb is 5/8 of an inch in diameter, a T8 bulb is 8/8 of an inch in diameter (or one inch), and a T10 bulb is 10/8 of an inch (or 1 1/4 inch in diameter). So the smaller the number, the smaller the diameter of the bulb, and that's important to us because the smaller the diameter, the more bulbs you can fit in the limited space above your tank, so the brighter it will be.
If you are getting new lights, I would definitely recommend getting them from a store that specializes in saltwater, I think
www.marinedepot.com and
Champion Lighting & Supply are two of the best, they have very good websites. If you have any questions, they have excellent customer service that can talk you through your options and help you pick one that's best for you.
Don't get me wrong -- I'm not saying don't go through Petsmart, Petsmart is a fine store too and they have excellent deals on stuff most of the time, but if you had questions about a product, I doubt the Petsmart rep on the other end of the line would be as helpful as the Marine Depot or Champion Lighting reps. Those guys seriously know their stuff, I hardly ever order anything important or expensive without talking to them about it first.
I hope I helped a little bit!