@Matt_W has some fair points about OnAir: the search functions aren’t (yet) as streamlined as they could be, but worst case it’s usually a couple extra clicks. And yes, there are fewer MSFS aircraft right now, although I’ve had no trouble finding rentals myself.
FWIW, though, rentals from other players are definitely not “forever”. By default, if you rent your aircraft to another player, it’s only for a couple days, and the UI forces you to pick some sort of return date. So if someone has rented an aircraft for a couple months, it’s their own fault. Maybe the UI feature that forces you to set a return date is new?
To me, OnAir is more interesting than FSE. It’s more of a complete “game.” I like having a visual plot of every flight I’ve ever done, including dots that record various in-flight events. (I wish MSFS had this!) I like that OnAir rewards me for good procedures (not stalling, beacon, flaps, landing/taxi lights, planning an alternate destination in case of trouble, landing in bad weather, etc). I like the idea of gaining XP, filling out a skill tree, and passing checkrides before I can use fancier aircraft. I like leaderboards that focus not on cash on hand, but rather reputation, return on assets, company value, outsourcing expenses, etc. I like having three worlds with three different rulesets, including AI pilots etc. I like that it’s relatively easy to get your own FBO running; in FSE, an FBO is a money pit.
To me, the biggest downside of OnAir is that it’s a subscription service. I’m still debating whether I want to pony up. If it were free, like FSE, it’d be a no-brainer for me. Also, for some people, OnAir is probably just “too much.” To play optimally, you have to be willing to start every flight cold and dark, and taxi at least at the start (though not at the end) of your flight. You don’t have to start cold and dark, but you suffer reputational penalties if you don’t. Also, I imagine some people won’t like having to pass a checkride before piloting anything faster than a single-engine piston land aircraft. I like that RPG element, but I know some players would hate it. I haven’t done a checkride yet; I don’t know how hard they are.
I’ll never give up FSE as long as it’s free. But FSE leans too far in the other direction for me. If I play it a lot, I start caring less about the flying and more about getting it over with fast so I can collect the pay.