There is a great deal of disagreement about various Model Ms. Many people believe that the older ones (pre-1989) are best, but there are those who prefer a new Unicomp. The 1391401 is considered the "classic" and just look for a clean tested one at a good price.
I would be inclined to recommend a new Unicomp, since you can get a smaller footprint, black (if you like that) and a native USB cable, in addition to fresh springs and a warranty.
The Model F is a completely different and more ancient beast. They are much rarer and do not come in a standard modern layout. Internally they are far more complex but you have some options when you take one apart.
There are those of us who feel that the F is the pinnacle of buckling spring excellence, but you make many sacrifices, not limited to search time and money, to acquire one.
Start with the Model M, and think long and hard about a Model F before you get involved.
