To put it in perspective its like being a sprinter and a marathon runner.
Um, not quite. It's not like he swam the 1500 free or anything. It's a marvelous achievement but swimmers swim multiple disciplines and distances all the time. I'm not arguing the fact that he may be the greatest Olympic athlete of all time, he may be and he's obviously the most decorated, just trying to temper the hyperbole.
I guess it also depends on what your personal idea is of greatest. I heard someone say Tommy Smith and John Carlos were the greatest. Kind of out there but they were certainly more important that Phelps. I'd also point out that there is far greater difference between the 100M and the Long Jump than the butterfly and the crawl. But for my money Jesse Owens has to be the greatest AND most important of all time.