Hi Sven,
We can be here for years discussing the benefits of Linux over Windows or vice versa. Like most things, it's not only a cost issue. (Lets face it, if everything came down to cost then we would all be driving KIA cars and shopping at Aldi)
IMHO, I just don't see FSX performing as well on a Linux machine as a Windows Vista machine. Then again, I know as much about FSX as you and most of the rest of the world at the moment. I'm sure we will see at the end of the year.
Just as a matter of interest, if you are running FS2004 on a Linux system how does it perform compared to Windows? Are there any differences in performance? I last ran Linux back in 2002 (and it was a lot more problematic than my Windows system at the time, if I recal) so I haven't got a clue how it is today but I know if I used any sort of Windows games (using WineX) it was slower and a lot more unstable than running it under windows? I guess this must have changed if you are hapy running FS2004 on Linux.