Realflight has much much better heli physics than FSOne IMHO. With the G3.5 updates that just came out I think the plane physics are probably about equal with Realflight being just a bit better. Overall IMHO Realflight is just a much more mature product. It's been around a lot longer. The advantage of FSOne is that the developers have been very responsive to the issues and there seems to be a desire to take it to the next level. Right now though I think RealFlight takes the cake between the two.

Another good one to look at is Phoenix. www.phoenix-sim.com. Tower Hobbies sells it. A little cheaper than realflight and fsone but it does not come with a fake transmitter so you have to use your own. It's got some advanatages though. The plane and heli physics are great and it also includes float flying which is something that realflight does not have. It too is new though and is going thru growning pains but the developers have been very reponsive and they have released a lot of new planes/helis since it was released free of charge.