I am a gymnast and some people say football is harder and some say gymnastics is harder. Both are my favourite sports.
I think if you compare a professional football player against a professional gymnast......that gymnastics is harder.
Gymnastics combines flexibility, muscle, perfection, balance, toughness, etc. They are doing all these things on a 4inch beam or flying high on those bars....it's crazy! I think gymnasts have to give up a lot more to go further in the sport because it is so select. Also, many people don't realize how professional gymnast are up at 4 in the morning and stay at the gym till late at night....it's a tough sport. And after all is said and done....very few gymnasts ever make it to the top.
It's also hard to compare because gymnastics is really more of an individual sport. I know they compete as a team in worlds and the Olympics....but success is really dependent upon you. Football is much more team-oriented...of course there are some stand out players....but no matter how good the players are if there isn't chemistry they are not always great!