The 42mm is the one that was in the movie. If being film accurate matters, then that is the one you should get.
Otherwise, you should go with the one that feels better on wrist.
I would go with the 42mm for the reason stated above. Since the 38 wasn’t in the movie, it’s just a Hamilton watch. I would get the newish Khaki Field Expedition instead of the 38mm Not-a-Murph.
They almost always use a Seiko automatic movement (or Ronda for their quartz models). That particular watch uses a Seikko NH34. While it may not be the prettiest or most accurate movement, it is robust and reliable.
I do think many owners/reviewers of San Martin watches tend to overstate the fit and finish of the watches. Specifically, while the brushing and polishing is very good, the San Martin bracelets I own have very "crisp" corners that are sharp to the touch. Overall, I put them on par with low-to-mid tier Swatch Group watches (Mido, Tissot) and not anywhere close to my Omegas, Tudors, CF Bucherers (or even my Rados).
Any company, including San Martin, can have a QC hiccup. One of the screw heads on my last San Martin watch broke as I was sizing it.
Nevertheless, San Martin watches are well constructed using quality parts and materials, and they should last a long time.
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