The reason I prefer the Customizer is (1) Don't care for the windows key, especially when I'm gaming. I guess this will change and I'll give Windows key shortcuts a try when the next time I buy a Topre. (2) I like that "traditional" look. That old-school, 1391401 look.
I'll admit that the Unicomps do have a somewhat "old look" but I wouldn't say the same for the 1391401. The other day I put the keyboard on the window sill as I prepared to ship it away. That keyboard glowed like the marble of the Taj Mahal! I know I'm in the minority but I consider the 1391401 to be a beautiful keyboard (especially love the Taj Mahal marble color and luster) and a classic, trendsetting design. I like the look so much that I ordered a sealed, NIB 1391401 just for the artwork as opposed to type on it. As much as I love the tactile feedback of buckling springs, I plan on moving away to lighter key switches like the Topre or Cherry.
I prefer Unicomp for the Customizer and will probably get the Topre tenkeyless as a Spacesaver. The Unicomp Spacesaver is considerably lighter than the Customizer. Yes, you will not be able to use it as a weapon. From a functional standpoint, it is pretty durable IMO and of very good quality.