How should I decide which flooring underlayment is best?
The question is quite broad, but there are two primary differences (aside from cost): sound management and the ability to level imperfect subfloors. 

Sound management (deadening, which can reduce tapping noise) depends on the thickness and composition of the underlayment. Leveling ability is primarily a matter of underlayment thickness. 

You'll need to discuss the options as they pertain to your choice of flooring and your subfloor situation with your sales pro and make a decision based on your personal criteria.