Having used both FMS and the newest version of G2, I can say from my own experience that G2 offers much more entertainment and versatility than FMS. That said, do not underestimate the value of FMS. It is a freeware software so your only cost is for the interface cable to plug your R/C transmitter into your computer, and for those without time or inclination to make the interface cable for themselves, you can pick one up on ebay from a number of dealers, for about $20. FMS has a gazillion computer program writers out there on the internet and the variety of planes and helis, available...for free...is virtually endless. G2 offers add-on disks of planes, and helis, to keep your interest up, and at $30 each, it's a small additional price to pay after you have already forked over about $200 for the full G2 program. The value in G2 is that in addition to a lot of neat gimmicks, you have the ability to select your preference of flying conditions. Thats not an available option in FMS. It's great to be able to practice your take-offs and landings in a crosswind, and develop your skills, without risking your real R/C plane on those real windy days. I have found that the crosswind strength you select should be a lot less than you think you need. The G2 wind strengths seem to be much greater than the numbers would indicate. With less than ten MPH, the plane gets blown right off the runway before takeoff. Good for a laugh, but you probably wouldn't be trying to fly under those gale force conditions. To sum it up, G2 is a wonderful and entertaining tool, if you can afford it, otherwise FMS will do most of what you need, and does it very well. Most flyers are well pleased with either of these simulators.
Happy flying.