Hey Stearman, sorry for the late answer.
Let me try to answer a few of your questions:
Hi, I've been a Microsoft FS user for as long as I can remember (I still am), but I feel the time has come to give X-Plane a try.
I'm somehow scared about the learning curve, but I trust you guys can give me some useful advice.
Plase take into account I'm only interested in GA (not airliners or military).
Here are some basic questions to start with:
Which X-Plane version should I go for?
Version 12. Always go with the latest, current version. There's no benefits to keep with ancient versions, excepted if you like keeping with outdated addons, outdated graphics, etc.... Xplane12 is XPlane11 with everything better in it anyways, so go with 12. By the way, keep in mind you can download a demo version of XPlane from their official website. That demo comes with a ground scenery limited to the area around Portland if I'm not mistaken, and is limited to 20 minutes of flight before you have to restart the sim (unlimited attempts). You can also install addons in the demo just like the full sim.
Finally, to switch to the full sim, you basically just need to buy a licence key, input it into your demo to "register/activate" it, and voila, you've got the full sim
Yes, the demo is not a "special" version of the full sim, it's really the full sim but with limited scenery and session time.
Once activated, you can use the installation program to add other areas of the globe to your installation (Earth is just a big ball of water per default, and you add regions/continents on top of it). Very easy in just a few clicks.
Hardware requirements are higher, similar or lesser than MSFS?
A bit higher requirements than MSFS actually, but not much. Performance/Visuals ratio is quite similar, especially since the latest versions/updates. No fear on that side, and again, the demo will help you assess the situation more precisely
Is X-Plane a sim you need to fly always on-line, like MSFS?
No. It just require some re-activation (just a popup asking for your licence key, big copy-paste) from time to time. Of course you need to be connected if you want to use real-weather.
Every two months (not counting hotfixes or minor updates), and it's not mandatory. You'll get an info that an update is available, but you'll need to relaunch your installer program to trigger the update, if you want. Again, not mandatory, you can delay if needed.
What's the scenery coverage: does it cover the whole World?
Whole world, but you simply need to select the regions you want to install. If you don't install a region, then you'll see only water in there. As if the continent didn't exist. For example, in my sim, I have only installed the West half of Northen America, because I only use XP12 to fly around Seattle and such, so I haven't installed Eastern America, or Southern, or Europe or Asia or anything else. Makes the installation lighter, and if I want I can install them any time.
What's the detail level of scenery?
It's not very good per default. However, there are many addons, freeware and paywares, to improve drastically the situation.
Keep in mind that the XPlane default scenery is generic, just like FSX or P3D, not like MSFS. There are no aerial photo textures (called "orthoscenery") per default in XP12.
What's the flight realism level? Is it adjustable?
It's very high realism and fully adjustable/editable in each aircraft parameters, a bit like editing the CFG files in the FSX/P3D/MSFS aircraft. Some aspects are lacking while other are shining, just like in MSFS. The water physics are pretty bad just like in MSFS. The helicopter physics are legendary though.
I assume you can buy extra aircraft to include in the sim. Same applies to scenery?
Yes, exactly.
I think that's all for now.
Thanks in advance, best regards.
Don't hesitate to ask if you want more precise info.