Avocado's Number wrote:
The mile isn't often contested in many parts of the world these days. I believe that there have been far fewer GMs than sub-4 and 1500m-equivaIent performers.
That, of course, does not answer the question about which is "harder." Although I believe that runners and chess players tend to have certain commonalities in personality, there isn't a lot of overlap between high-level runners and high-level chess players, and probably not a huge amount of overlap in high-level talent in both endeavors. Offhand, I don't know anyone who has achieved the GM title and run at a 4-minute-mile level or better, and I don't recall anyone who has gotten very close. I do believe that a highly-gifted runner can develop into a 4-minute miler with fewer hours of training than is required for a highly-gifted chess player to develop into a GM-level player. Again, however, I don't think that this provides an especially satisfactory measure of difficulty.
By the way, a few people have mentioned the smaller age window within which one can develop sub-4 ability. It may be of interest that, although chess players may be able to reach a high level at a somewhat lower age, and may be able to sustain a high level at a somewhat greater age, the peak age for most top-level chess players isn't much different from the peak age for most top-level runners. Very few top chess players continue to improve beyond their early thirties, and most have significantly diminished skills by the age of forty. Among world-class players, most are under thirty. Only about 10-12% are over forty, and almost all of them were better when they were ten years younger. Also, while I am familiar with some excellent runners who did not compete until they were about twenty or older, it is very unusual to come across a GM-level chess player who was not immersed in the game as a teenager or younger. Essentially, if you are not a master-level player as a teenager and an international-class player by your early to mid-twenties, you are unlikely to ever become a GM.
Quoting thks on page 2 since Randy's s data dump kind of messed up page 1.