I've heard rumors that they're working on missions or some kind of game on top of the sim. That's awesome! I really hope they do something cool.
As for OnAir, since I'm still waiting in FSEconomy account creation limbo I decided to give OnAir a go. If you register with them you get a week free, so at the very least it'll keep me occupied until I get my FSE account. OnAir is definitely the slicker product from what I can see. The interface is much nicer than FSE and the integration with MSFS is very cool. I appreciate the very detailed management sim that OnAir have created and I can see myself enjoying playing the 'air freight tycoon' role for a long time. It's exactly the kind of activity that I would need on top of MSFS to give flights meaning.
From what I can tell OnAir and FSEconomy are very similar in what they're doing for you, with the biggest differences being A)Price , B) Age of codebase and active development, and C)Size of community.
FSE is a 15 year old product and I'm not entirely sure how active development is on the core product. Their GUI looks dated though, and their website and account creation process is real 'spit and chewing gum' level. But.... it's free. OnAir, by contrast, has a very modern look and it was a snap to create an account and dive right in. But, you have to pay. I've also heard that FSE has the larger established community, which makes sense given it's been around the longest. I suppose if you really want the MMO aspect of a management sim (renting/buying other people's aircraft and doing their missions) then FSE is the way to go. OnAir is more solo.
I should get my FSE account today or tomorrow if their progress in working through the backlog holds. Then I'll be able to compare properly.