it depends on what you want, if you want a program that add failures at random without you knowing what will fail, an scoring system for your flying skills, and the feel of real passengers in the back then you can't go wrong, i use FSpassengersX for every flight i do (excpt for reviews) and love it, you can even do sight seeing tours, and aerobatic thrill rides, apparently if you get too rough with the aerobatics, you get a rather realistic throwing up sound!!!
it teaches you to fly within your aircraft specs, and also make you think about the consequences of your flying, personally i don't know how you can get a full 747 load of passengers to fly out of baghdad, every flight i have had out of there i have been shot down and made an emergency landing, and the passengers were pissed because they crash landed, war zone people!!!
i recommend this. can do up a proper review for it this weekend if i get time if you are intrested
Jason