As a registered user of BOTH Notmad and MC 10, I feel like I can speek to their pros and cons.
In terms of basic music transfer and synching functionality, I consider both products to be equal. I originally bought NotMad during a period when MC's Nomad plug-in was a little buggy. I don't remember which specific bug was a deal-breaker for me (I think it had something to do with MC not transferring tags properly in WMA files), but at a certain point, I said, "screw it" and bought Notmad. Since then, MC's Nomad plug-in has gotten much more stable and full-featured, and I'm happy to use both.
The main thing that Notmad can do that MC can't is stream files from the jukebox in real-time (i.e. without downloading them to the PC first). Any media player that supports HTTP MP3 streams can play the files. I use this when I travel to play files from my jukebox over my laptop's speakers.
The main thing that MC can do that Notmad can't is synchronize your playlists. Technically, Notmad does support playlist synchronization, but you have to manually export them from MC first and then manually import them into Notmad. With MC, it's one click and all playlists are synched. This is especially handy with SmartLists, since they can be easily updated every time you synch. This feature is why, even when I use Notmad to transfer files, I use MC to synch my playlists.
MC has the potential to do things that Notmad hasn't even thought about. One example is generating tags at synch-time based on other tags' values (for example, "Album" on the jukebox is "Album Year - Album" from MC). MC's nomad plugin can't do this today, but I'd lay good odds that it'll do it before Notmad does.
Another advantage of MC is that you can use it to synch for free, whereas you have to buy Notmad separately. If I hadn't been compelled to buy Notmad by earlier bugs in the nomad plug-in, I certainly wouldn't buy it today.