This is like starting a thread to find the meaning of life. If you were to ask this question in the FSX forum, most would ask the question. . ."why would you even want to try". Asking the question within the FS9 forum and you'll get a lot of folks who have either tried FSX and gave it up due to poor fps or found it "too cartoonish".
I have flown both and would probably be flying FS9 more because I have a ton of great FS9 aircraft both freeware and payware and almost every scenery/mesh/terrain enhancement possible and love the way it looks. I am currently flying FSX and have to say that FS9 will never look as good as FSX. Yes the waves seem out of proportion but even given that, they look far better than anything I have for FS9 (that includes payware addons that still fall short). Another thing that is default in FSX that requires an enhancement program to even comes close is how the Cities (large and small), towns, villages, hamlets, etc, etc. are laid out. Very realistic as opposed to the helter skelter method used in FS9. AI is also another plus you'd be hard pressed to pull off in FS9 (darned near impossible actually). In particular, AI road traffic. Very cool to be flying low and slow over farmland, small town or big cities and see the highways and side streets and farm roads populated with traffic.
I enjoy both Sims, each has it's place for different reasons and everyone has a favorite and they're usually rather passionate in that respect and that's great. I don't care one way or the other who flies what or why really. . .FS9 will be around for a long time to come as will FSX and as great as folks can make FS9 look, it won't ever compare, overall, to FSX.