Basically what the title says. I haven't SGed for Ars Magica before (in fact, I've only even played three sessions because the SG got the flu for a few weeks and the game never picked back up) but I'd like to try. What are the pros and cons of SGing for the first time over PbP, compared to finding some players IRL (the most experienced people I know have played the same amount as me, so basically newbs) and running sessions for them? In which ways are one method better or worse than the other for getting me used to the [strike]prestigious[/strike] complicated job of being a Storyguide for Ars Magica?
I've never played Ars on Pbp, but I have played many PbPs in other game system, and the biggest limitation has always been the glacially slow pace. Anything you can do to mitigate that is a good thing.
Running multiple plot lines at once can help. Have each player be a Mage in one thread, and companions and grogs in the rest of the threads and run them all in parallel. Once you finish up a storyline consider jumping ahead a number of seasons.
PbPs are indeed glacially slow. (This is often especially so for combat.) They are also very prone to player abandonment, much more than face-to-face games. Recruiting for them can be easier, but expect to lose lots of players.
It's also somewhat of a different experience. A big fun of gaming with a group, for me, is the humor that ensues, and the personal back-and-forth. This just doesn't really happen in PbP, and much of the drama is lost too as it's spread over too great stretches of time.
Running multiple concurrent threads is a lot of work, though. I did it for Bibracte, which had nearly 20,000 posts in two years. I found I couldn't put plot points on characters, because they would suddenly disappear. It was challenging writing posts for players who wanted to cooperate in a story but had vastly different posting rates. So in threads where more than one character was in play, the pace of that story was determined by the star of the story, be he a magus or a companion.
A very big pro is that it is a lot of fun to write, and I found I was able to progress a story in about 6 weeks, so I tailored stories to the players' posting rates and timed it for 4-6 weeks. Towards the end I was spreading stories out for individual characters over two or three years at a time to find the right mix of progressing characters (which is kind of the point of Ars Magica) and also have characters do stuff.