Hehe, although I am a die-hard FSX flyer, I have to agree with Alexander
Stick with FS9, for now, and give X-plane a try
I fly only FSX, because that's what I started out with, and I could not get used to the difference in keystrokes required to look around in FS9. Also, I got used to flying VC, which is not common in FS9, so I would miss that. Reason for me to stick with FSX, despite the whack fps... (which is not really a problem anymore since I bought me an i7-860, 6GB, GTX-260, W7-64 machine; locked @ 25fps no problem)
So I'lll stick to FSX, at least until a little after X-Plane 10 comes out. I have the demo of X-plane 9 installed, and I can advice you to do the same. It's free, and gives you a good idea of what to expect from X-Plane 10, if you know how to imagine what improvements will come with a next generation. X-plane has a very different "feel" to it, is incredibly fast, fps-wise, graphically comparable to somewhere between FSX and FS9 (X-Plane 10 will be MUCH better in that area), and it is said that it is more realistic as far as flight dynamics are concerned. I have next to no real life flying experience, so I cannot judge that, but it is certainly more demanding to fly in X-plane than it is in FSX.
To get back to your original questions:
1) Larger airports are OK, no addons needed, unless you really want them to be detailed as in real life (like EHAM, LFPG, EGLL, LPPT, etc. have marvellous addons). Regional airports, such as Castlegar are OK too. Only the realy small strips are not much more than that.
2) General scenery: not bad at all! see
for Swiss Alps in default scenery, for example.
3) The folder structure is slightly different to FS9, but other than that, no significant differences
So, the choice is yours, really. Stick with FS9, try out FSX, switch to X-Plane ...
But considering that the MS series will not continue to be developed any further (only the addon market), my advise would be to (also) give X-plane a try.