FSX vs FS9?

OK - this is not a "can my system run X" post - I got a copy of FSX from a friend who's PC wouldn't run X, and my system can run it reasonably well. I can run med-high to high on most sliders, even ultra-high on a few, fps locked @ 24, and I get decent performance.

However, I know my system would scream on FS9. Now, i've never used MSFS before, so I have no idea the differences. I know there are tons of cool add-ons for 9 (esp. fspassengers, which looks awesome), but what would I *lose* by going back to the lower version?

For example, moving jetways, road vehicles, etc... Well, they don't mean a lot to me. What does mean a lot is nice looking scenery, realistic flight dynamics, smooth flying, etc...

I looked at the "What's new in X" stuff on M$'s website, but I want to hear from you experienced pilots and simmers - what reasons would make you jump up to X? What if the performance wasn't an issue - would you give up FS9 with all your customizations for FSX?

My basic question - should I continue running X @ high settings, growing into it? Or shelf it until the day comes a few years from now when I can run it all out, using 9 in the meantime? Or is it one of those things where, once tasting X, I wouldn't be happy moving backwards?

Any opinions welcome!