Personally I could never go back to offline flying. The Vatsim network is great and provides a realistic experience. Especially if you are with a VA (which I must add, ISN'T a necessity when you're on Vatsim).
Sure, you won't see the number of aircraft that your Traffic program can replicate, but nothing beats talking with a live controller, for realism.
There are too many VAs to even mention here.
Some are based on real airlines, other's aren't. Some are super-strict like BA/Scandinavian (in fact, Scandinavian Airlines require a flight video checkride for you to be accepted! ) And there are others that are leniant. Some support payware models, etc etc, etc. It's endless!
Just have a look around.