I like to do both, it lets me have fun with it and also keep things in order and control the world.
Of course I have no experience as a GM... but I can imagine.
Some people can't really not be biased towards their own character though. One time, I was in this Pokemon RP, and my rival was the GM. He controlled my character in his favor much more often than not. I kinda dropped out of that one, but still. Not everyone could do it, they're better suited for one or the other. Otherwise, though, being both is a good experience.
This literally happened once in said RP:
I'm winning a battle against him
GM makes his starter OP and controls my starter
Suddenly it ends in a tie
pokemon lazuli • coming soon