I'm a convert to x-plane (mac and windows versions) and can vouch for its fun factor as well as its accuracy in flight modelling. Switching planets to Mars (!) and having options for setting space weather is kind of interesting too!
However FS2004 got a great review in this months Aus Aviation.
My inclination is that your computer platform may be the decider. Its hard to get a ripped off copy of X-plane so if you are windows user, might be an easy decision.
The comment above about 'sufficiently speedy computer' is interesting too -- my eMac cost me around $1800 new and it runs x-plane very well. A very modest hardware overhead!