Another thing to consider is what the pad is made of. The most common and most affordable is called rebond and it is made up of pieces of different types of foam. This is the kind that most people use and is a great all around pad. Other types of padding include rubber, felt fiber and prime. Each has their own advantages and disadvantages so ask your salesman for details.
Lastly you should consider if you want a pad with a stain barrier. Stain barriers have a shield that fluid can not pass. This keeps you from getting those pesky stains that keep wicking back after carpet cleaning. This is especially useful if you have pets that might have accidents and you do not want the pad contaminated.
As you can see, there is a lot to think about when choosing a pad. Give it some thought and choose the one that makes the carpet work best for your situation.