Well, since I have been through both classes, I might be qualified to answer this question, but that still remains to be seen...
Anyway, NAUI's master diver program is very complete as far as rec diving is concerned. We did some advanced Navigation (more than just out here, turn there, now go back), search and recovery, we did an environmental clean up dive, we did a few night dives, we did some deep dives, we did a lot of situation handling exercises in the pool (lots closer & cheaper than the quarry).
The book for the class even gets into different regulator designs, pros and cons of reel designs, reasons for taking safety equipment, what shoulkd be in a first aid kit & save a dive kit.
Tons of info related to just diving in the Master diver course.
The DiveMaster course focuses mainly on rules, regulations, and standards set forth by NAUI. You have to work with classes, you have to do a lot of research (well we did anyway), and you don't do a whole lot of diving. Being a DM is more about setting up and preparing dives for groups of individuals. You have to give dive briefings, you have to give dive reports. You have to be able to follow the steps and report events to emergency responders in the event of an emergency. You have to know who to call, you are in charge.
So conclusion question- Do you want to dive, or do you want to work? Frankly I think the Master diver class is a great step to getting to DM, but I don't think it is a NAUI requirement. It sure makes your job easier knowing what kind of dive your going to be planning for, and it gves you more experience in caculating what someone elses diving ability is and how to plan a dive around that by going through the master diver class and doing loads of dives with others.