I was in cheerleading since I was in 5th grade. I also competed in a few pageants during high school (all where for scholarships and/or community based). There is a HUGE difference between the two. With cheerleading, it is highly physical, especially a competitive squad that is mostly stunts and tumbling. Also, it is a team and if you don't win, you have your teammates to be there with you. And if you win, there is a amazing feeling of togetherness with your team. Basically, you have to be talented, not just beautiful. And yes, you can start cheerleading around 3 or 4 (for Pop Warner teams!)
With pageants, there is A LOT of fakeness behind the stage. Everyone secretively wants the other to fall down, their dress to rip, or their hair to turn out horrible. While I enjoyed my experience and did make some lifelong friends, it was a different feeling than cheerleading. Then again, I was 16-18 and chose to participate on my own (my mom actually laughed at me!).
These girls, especially the really young ones, are pretty disturbing. And their parents are even worse. Some are just plain crazy! But it does make for interesting TV!