I would recommend to read up a bit more on the various types of speedmasters before pulling the trigger on buying one. Unless you are going to buy one of each, then it probably doesn't matter where you start.
In a nutshell, and horribly abbreviated, it makes sense to group them by the movements they use:
Among the manual winding speedmasters there's original 321 based ones from the 1960's, speedmasters with the 861/1861 series of movements from the late 60s to a short while ago, and recently 3861 co-axial handwinders replacing the 1861s.
For automatics there's various ETA movement based ones (2890, 7750), and for the last ten years or so various co-axial automatics (9300, 9900 etc).
Manual winding ones are typically thinner, early automatics were smaller diameter, and the recent co-axial automatics are usually pretty large in diameter and height.
But there's whole books about speedmasters...