I think the most desirable full-sized Model Ms from a strictly objective point of view are 1391401s made by Lexmark (1992-1994) - that way you get the two piece keycaps, removable cable, but also the modern trimmings such as drainage channels and the additional internal supports.
As for the space-saver Models, it's pick and mix. I've seen a few Lexmark space-savers that don't have drainage channels... And there seems to be no way of knowing whether you get double or single piece keycaps without asking the seller. Either way, they're good no matter when they were made, or which part number they had.
For more info on part numbers, the IBM Wiki has a detailed guide on the common ones.