I’ve done all of the above. IMO, the 50 mile run community is smaller than the IM triathlon community. Double century is probably somewhere in between.
As for which is the more difficult event, it really depends on how much you’ve trained and how fast you plan on going. The training commitment to do a 10 hr IM is the most time consuming but the average joe doesn’t know a 10 hr IM from a 17 hr IM (and doesn’t care).
How common? Not very. But pretty much anyone who does one of those could do (at some level), and, I think, enjoy (at some level) either of the other 2 (modulo the swim requirement for an IM). In my experience they’re all three about equal (more or less, depending on specific preparation and goals) in basic physical difficulty. Definitely slightly different personality populations, and different mindsets needed for each. Double riders and ultrarunners seem more closely aligned personality wise than either are with at least the current crop of Ironman triathletes. If you’re trying to collect actual anecdotal data, I’ve done…6 doubles, 6 IMs, 6 50’s (and 3 100s), along with a bunch of 50Ks and non competitive centuries, spread over about 20 years, so I guess I have been attracted to them all equally, at different times.
2 IMs
4 DCs (one the week following an IM…which was a bad idea)
Never done an ultra run or double/triple IM, but I’m looking to do one of them next year.
Furnace Creek 508