Okay, first we need to clarify something. Are you talking about PADI courses? This is significant, as PADI doesn't *have* a Master Scuba Diver course. There is a PADI Master Diver *card* that you can get by meeting some prerequisites, but there is no *course*.
Whereas in the NAUI progression, you can take the NAUI Master Scuba Diver course and learn a great deal, in the PADI progression, such in-depth information is only available in the PADI DM course. PADI DM is a sort of recreational/leadership hybrid course. Some people take it for the knowledge, with no intention of leading, while other people take it as a step toward leadership roles. NAUI puts all the non-leadership knowledge into the recreational NAUI Master Scuba Diver course and then works solely on leadership-type skills in DM, which I personally believe makes DM much less bipolar in the NAUI world than in PADI.
Anyway, so the question really becomes "What are you trying to do at this point and in the future?" If you're just trying to increase your knowledge and skills, it's far better to go with a non-leadership course. NAUI Master Scuba Diver, GUE Fundies, and other courses can often greatly expand and improve your knowledge and skills.
On the other hand, if you are planning to go into leadership, you should take a step back and have a serious look at yourself and your diving. You can even ask some mentors about it. Are there any missing pieces in your diving so far? Using a spool/reel to shoot an SMB, playing around with lift bags, working low-viz navigation, holding no-reference stops, diving locations far different than your usual, or being the "buddy in charge" on dives or even trips are some examples of things with which a diver may not have much experience, even with many dives logged. Anyone planning to go into leadership should have a broad base of skills and experience already. (Those who do not quite often end up being the zealots who have The Law without any understanding of why it's there and when it may not apply.)
If you take an honest look at yourself and see that there are areas you find yourself lacking, just start building up those thin spots. You can take a course (such as NAUI Master Scuba Diver or the others), which is a good way to get a broad foundation, or you can just work on things on your own if you're good at self-study.