Both are awesome, but the Iracing is based on the 2009 car and the FVA based on the 2010 car. Now FVA feels more understeery, however one of the complaints of the real life 2010 cars were that the new tyre profiles made the cars feel quite understeery. FVA is based on netkar which is thought to have the best tyre and force feedback model around, it was also developed around mainly open-wheel cars. Both are really awesome but the general opinion is that the FVA simulator is the most accurate, even on the IRacing forums, and for these reasons.
Braking : FVA acts just like the real car, the technique needed to brake properly and the reaction of the car to the brakes, the way the tyres lock, learning the brakes in FVA is an art and one of the big challenges to mastering the car. IRacing isnt bad by any means but the way you brake in the IRacing car is not quite the same as in real life, looking at telemetry of F1 drivers braking patterns shows a style that works perfect with FVA but in the IRacing car would result in a lot of locking up and skidding, as less brake pressure is needed and you rarely, if ever use full brake pressure in the IRacing car.
Throttle traction : Some people have said that FVA has too much traction, though the reality is if you compare it to what you see on the tv, and you look at telemetry its pretty much perfect, the IRacing car is also quite close on this though the back end is a little more unstable, the instability seems like a common thing with the older software and a car built around a physics model that is being adapted for the F1. You can feel the traction point a little better in FVA and it reacts to driving that is more similar to what you would see the real F1 drivers doing, though by all means the IRacing is very close too and with good setups you will see some very lifelife laptimes and driving.
Though the FVA is by no means perfect and many people would probably enjoy the IRacing car more.