Age of Worms!!!
I'm running Shackled City IRL and would really like to play an Adventure Path as a player. My preference would be for Age of Worms since I'm sure my friend will want to run Savage Tide since he's a pirate freak.
I understand the motivation to run a short game, but I would prefer a longer, more involved story. It's true that the same 4-6 players might not all have their characters retired at Level 20, but as long as the posts are frequent and the game interesting, I don't see why it can't go on as long as the DM wants. The Adventure Paths are neatly broken down into 12 self-conteained scenarios which means there are a lot of natural break points where players can be recruited to replace dropouts.
I have a few suggestions to help make a long campaign work:
1. Smaller party (preferably 4 PCs, 5 max).
This makes everyone feel like an integral part of the campaign. I find with larger parties, a few people tend to get lost in the shuffle or intimidated my the quantity of posts they have to read.
2. Restrict to core material
People get over-excited about cool, exotic, powered-up characters. This is great at first, but it quickly loses it's luster and often leads to bored players. Some people just like making characters. I find that simple characters are more conducive to good role-playing.
3. Visual Aids
I find this really helps in PbP. Even if it's just a cropped photo from the adventure or a nice-looking battle grid, it's nice to see what's going on every once in awhile rather than have to scroll back through old posts for a description. Seeing that the DM is putting it that extra effort is also a great motivator for players to log on every day and post.
Anyway, it sounds like you'll have interest no matter what you run, but that's my vote
Jon