Pro Photographers: Which is harder, in your opinion to master? On location or studio?

I don't know if one is harder to learn, but there definitely is a difference in available resources :) That said, I do on location work most of the time and use my studio more so as a gallery until the cooler months. I work alone on location sessions only when I have to. Carrying all the equipment you might want to have available (just in case) could put a drain on your quickly. And watching the wind toss your scrims or reflectors around just isn't cool. So, I learn to shoot differently due to the different resources available. But the knowledge needed to shoot differently is the same you'd use in the studio.