Hi,
The current Macbook and Macbook Pro will run both sims pretty well. I’m a recent Mac convert and a big fan of X-Plane. To run FS-X on a Mac you would need to use the bootcamp software (which comes as part of OSX) to install Windows on a seperate partition. If you don’t want to install another operating system then X-Plane is your only option to sim on a Mac.
Both sims have pros and cons, FS-X is graphically superior, X-Plane has (in my opinion) a nicer feel to the flight model and smoother guages but doesn’t look as pretty. The only thing I miss on my mac is native support for my Track IR head tracker.
Have fun!
Bruce