It's of course a great question and one that existed forever.
Having actually competed in an Olympic Games I always thought the answer here was an easy one - an Olympic gold medal, but an earlier post pointed out that in winning an Olympics you win one important race vs when you hold a WR you can lay claim to being the best in your event to ever do it.
That's actually not a bad argument at all, and in thinking about this it opened up so many factors I never thought about before.
The argument given "who would you rather be - Kipketer or Rodahl" is a decent one, but it is all subjective. Of course the answer here is Kipketer but he is an exceptional case for someone that never won the Olympic Games. I mean he still has an Olympic silver and bronze, 3 world titles and dominated an event for basically a decade.
On the other hand would you rather be Sydney Maree or John Walker? Maree was the World Record holder in the 1500m and Walker "merely" the Olympic Champ who never even broke 3.30? I think we all know the answer to that. How about Peter Rono or Steve Cram?
The reality is not all Olympic gold medalists are made equal and neither are world record holders. There is so much more to the resume of WR holder and to an Olympic champion to take into account before making that assessment.
I believe the general consensus amongst elite athletes is that the Olympic Gold medal is worth more. You can hold a world record but the moment it's broken you become second and can only go one direction. When you win the Olympics you remain (a) number 1 forever. The Olympics hold more prestige because they are the epitome of performing better than anyone else on the planet over a 1-4 day span that every single athlete knows about 4 years in advance and targets. You can in theory rock up to a club meet any time of the year and run a WR - planning the greatest (or close to) form of your life for such a small window of time when everyone else is doing the same - that is hard and that's what makes it special.
Back to Kipketer - I know first hand that Wilson would trade his WR for the Olympic Gold. He told me. He knew his WR would one day be broken. He didn't need the WR to know he was momentarily the best in the world at what he did or that he was close to (if not alone) the best to ever do it. That's not to say he didn't want the WR because he did - I know he just wanted the Olympic Gold medal more.
As for me? I guess I was closer than most only because I had both the opportunity to run a WR (like anyone alive does) and win the Olympics (a chance that only a very select people are lucky enough to have) because I actually toed the line - I was pretty far way on both counts compared to some. But if I had the pick I'm taking the medal too.