Re: When to GM and not GM
I think it's also important to acknowledge your GMing style.
I, for instance, rely on props and tools and devices that just don't work in an online, PbP setting, so I confine my games to tabletop. I also tend to run either short story games, or episodic games, with breaks in between, so I don't lose motivation and energy during that long middle plot.
Some GMs can sustain the long haul, and breaks in the story break their game. Some GMs really thrive in the environment of PbP, and their bags of tricks work here. Some GMs can run amazing, tightly run one shots that end with a bang, and others can plot over years of campaigning that burn slow but intense for the whole time.
Running the type of games you do best at, the ones you have fun running, and the ones that spin out the best games is the best way to GM. It's important to acknowledge that having preferences isn't the same thing as being "bad" at something, and running the best game in your own preferred style is almost always going to be better than trying to run a game in a style you know you hate.
That said, it's worth experimenting with different approaches to different styles, but it's also good to be up-front with your players. If that style isn't working out, pull the ripcord and refigure, or step out from behind the screen to regroup.