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I vote for Hillary's, since running a 4 minute mile did not require courage to face the (realistic) possibility of death.
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Good point, although (eg) driving a car at high speeds through traffic has a strong possibility of death, and isn't necessarily a greater accomplishment than (eg) composing Beethoven's Ninth, which has very little danger.
Since both Hillary and Bannister accomplished their feats around the same time (seperated by about 12 months IIRC), it might help to look at the number of people that have climbed Everest (~1500 by 2001 according to wikipedia) vs. the number of people that have run a four-minute mile (~955 by 2003 according to [1], though I'm not sure "official" data exists on the latter.
[1]
http://www.career-in-your-suitcase.c...H/barriers.htm