First things first. You can remove RealFlight X from the equation. RealFlight X was their try to move RealFlight to a new generation of graphics, but it unfortunately failed because it was "very" demanding on the graphics card (at the time). You literally had to have the most expensive graphics card to run it decently. So most people hated it and it was finally given up on. And that's too bad because in a way it's their best rc simulator. The graphics were a huge jump ahead and when they came out with the last version of it (1.5) it showed great progress and is still popular today with lots of folks. But it's a dead product, when Horizon Hobby purchased RealFlight they didn't buy RealFlight X so it's dead.
Having said all that RealFlight 9.5 (not 9.5S) is the last RealFlight from KnifeEdge Software and is actually what I still use to model new planes. When Horizon Hobby bought RealFlight they released RealFlight 9.5S and offered a free upgrade to it, the "S" just meant Steam and it had a few extra features too. I never upgraded to it and am still using the DVD version of RealFlight 9.5.
RealFlight Evolution is the latest version and is only available through Steam and comes with almost all of the aircraft and flying fields in the Expansion Packs plus others, so it's a very nice version and is what I release all my new planes on. They said when they released RealFlight Evolution that it would be around for quite a while with upgrades to it as time goes by. It was the first "64 bit" version so it's not compatible with older versions. But they are holding to their word and are currently working towards releasing a DX11 version of it, which is a HUGE deal and will be a very good move for them. So RealFlight Evolution is truly a version that will Evolve over time and be great for them. I definitely think it's a good move to upgrade to it if you have the funds.