Compared with FSX, I much prefer to use FS9 because of the aircraft I have in FS9. Panels are better than FSX with all indicators easily viewable. I've tried moving some FS9 planes into FSX but the panels get plastered over FSX panel graphics and they move with pitch input - a real mess. Of course, FSNAV, which is absolutely essential for sim navigation, can't be used in FSX.
Since the only reason anyone would use FSX, is that they want the better scenery and terrain displays. The question is - is their any way to successfully delete existing scenery and terrain in FS9 and then copy the same from FSX into FS9?
I think if the makers of FSX had taken a vote from existing sim users it would've been in favor of just modifying FS9 with FSX style scenery and terrain and where we could keep all our existing aircraft and retain FSNAV. Give it a new name but indicate that it's a super modified FS9 but that all the old features of FS9 are retained. The sales probably would've been better than FSX sales.