I think that it depends on which version of each you are considering, given there are several of each. So; new or used?
Personally I prefer the 36mm version of the Explorer and the 40mm Explorer 2. The recent bigger versions are just too dammed big in my opinion. On the SM300 front, assuming your talking about the vintage inspired and not the Professional, I didn’t like the straight lugs.
Of the three, I have a leaning towards Explorer 1. Looks classy in all walks of life, dressed up or down. Doesn’t shout “look at me”, simplistic and functional.