It's OK. Questions are good.

For most platforms, CVS versions only come as source code which you have to compile for yourself. However, here on the Mac we're lucky and have lionel77.

He produces binaries for us, which is another word for an already-compiled application. It's just like the one you've already downloaded, just based off of more recent work. There's no complicated work on your end. Just double-click it and go.
The difference between the "debug" and "noDebug" versions has nothing to do with whether or not we've worked the bugs out. Functionally, they should be pretty close to identical. The "debug" version has a bunch of information embedded into the program so that, if you get a crash, you might provide meaningful feedback to us so that we can figure out what's going on and fix it. The "noDebug" version does not have this information. So, if you get a crash you won't be able to provide much meaningful feedback and we will have a hard time knowing why it's crashing. The only advantage of the "noDebug" version is that it's a smaller download.
As for recommendations, it depends a bit on your personality. The 2.0.3 release should be pretty stable. The more recent CVS versions has new features but
may also have stability problems and bugs, although lionel77 tries to pick times when things are working well to build his binaries. Also, 2.0.3 isn't perfect (we wish!) and lots of known bugs have been identified and fixed in CVS. So, the choice is between using a stable version with old bugs, or a CVS version with (probably) new bugs. Personally, I would recommend that you try a CVS version.
That brings us to your other question. You
can go back to 2.0.3 if a CVS version isn't working well enough for you. However, there are some issues created by the fact that aMule's configuration directory has moved. The CVS versions are smart enough to know about the move and transfer your existing configuration directory to the new location, but obviously the 2.0.3 version doesn't know about the move. So, moving forward from 2.0.3 to CVS, or one CVS to a more recent CVS, is safe. Moving backward requires a little work on your part. If you want to do that, you can read about the procedure in lionel77's thread about the CVS binaries, or come back and ask for help.