Basically, the frames are the same, but the components are better with each bike you are looking at. For example, the 7500 has better forks, better shifters, better Derailleurs, and more speeds than the 7300. It is less expensive to buy the upgraded components already on the bike than it is to purchase the items seperately and add them. The only real exception to this is if you wear the components out and need to replace them anyway.

Yes it is worth spending the money for the better components. But how good you want to go depends on your needs, wants, and available cash. Therefore, deciding how good a bike you are going to buy is a personal decision.