Is there any reason why you cant do both?
For me, it's mainly that I don't have the time to both play and run modules for two different Organized Play programs, and I'm much more attracted to 5e D&D than I am to Pathfinder, so I do Adventurers League. I could possibly play in both, if I felt the desire to spend hundreds of dollars to ramp up on my Pathfinder rule book purchases (I have the core books, the Kingmaker adventure path, and a couple of other books given out as swag at GenCon), but if I'm playing, I generally want to run games, too.
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Pauper