I have a lexmark made 1993 model M with dual keycaps and it's a very nice board. I have a Customizer 104 from Unicomp and it too is a fantastic board. I prefer the Unicomp for a variety of reasons. The main one is that they keys feel more crisp, as others have said, and the board is less "pingy" and more "gravelly" if that somehow explains it. I'm not sure if the difference is due to break-in time and wear, but there is something a little tighter and more precise about the Unicomp that I prefer. Both boards are great, though, and if you close your eyes, you really would have a hard time differentiating between both.
If I had to do it over again, I probably would just get two Unicomps and forget about collecting M boards. The build quality differences aren't that big of a deal to me (and only really an issue if you cant' stop staring at the board and insist on lifting it up, poking at it, pushing on the plastic, etc). The fact of the matter is that the Unicomp is an amazingly well made board and simply destroys 99 percent of the boards you can buy new on the market.
I also like the idea of supporting Unicomp by buying new boards.