Great question. I have the Railmaster and Speedmaster and love them both. The Railmaster was a no-brainer for me because I like simple watches and had been looking for an older Railmaster for a few years. The 60th Anniversary version is a very low key, understated watch but that's why I love it -- so many watches today are bigger and bolder. The Railmaster is distinctive for its simplicity and subtlety.
I'm not a huge chrono guy but I owned a Speedmaster Professional 3570.50 a few years ago. I liked the watch, especially the rounded Hesalite crystal, but sold it because because I found it a little hard to wind, due to the crown guards. The 60th Anniversary version looks even better to my eye, has no crown guards, and is easier to read, at least in one sense -- the chrono hand is slimmer and looks very different from the broad arrow hands. On the other hand, the 60th Anniversary broad arrow hands do get in the way of the subdials more than the slimmer hands of the Professional.
For some reason, the Seamaster was my least favorite of the three, but I do like it and would love to own one at some point.
From my own inquiries, the Speedy is the hardest to find, while the Railmaster and Seamaster are still pretty easy to find.
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