Both.
If you are happy with FSX then there's no reason to immediately change.
Neither X-Plane 10 or Flight are out yet, so we can't really comment on either, although I must say I am more excited by XP10 than Flight right now. Flight will probably be closer to what you're used to with MSFS, but will likely have many of the same issues - XP10 is having a more complete in built ATC out of the box and their default planes are getting a serious upgrade, the weather system looks exciting too. I'm expecting that FSX add-ons won't work in Flight, whereas XP9 add-ons will work ion XP10.
Lots of add on companies are seriously looking at X-Plane, so I think both sims will be good to fly with, although I am sure they will probably continue on their strengths.
The feeling of flight / fluidity will be with X-Plane 10 and eye candy will probably be with Flight, although I do not think Flight will enjoy the monopoly that Microsoft has in the past as the trend indicates a rise in the popularity of XP10. For serious simmers like us, I think X-plane is looking to be a solid option and I think what makes it exciting is that despite having a smaller number of add-on developers, it appears that the pace of development is faster in x-plane.
Take a look at some of the videos to compare what we know right now.
Flight (Be aware that pretty much everything released on Flight has only been on the Island of Oahu in Hawaii. Also remember to DirectX 10 'preview' images they released? Some of the Flight footage released appears to have been pre-rendered rather than 'in-game')
X-Plane 10 (Note that most vids show XP10 models running in XP9 so none of the XP10 effects such as shadowing, global lighting etc)
Default 747-400:
ATC:
Timetapse ATC controlled takeoffss - compare the physics of the planes to this:
More XP10 default aircraft mixed in with some XP9 payware:
The Payware CRJ-200 startup:
