The robbo is really twitchy in X-Plane. The S61 is nice to fly. I bought Jason Chandler's add on Augusta 109 and Bell 430.
The benefit with X-plane is the additional feature of equipment/instrument/engine failure sim. If you try to cut the engine in FS2004 the chopper just spins, whereas in X-plane, it reacts as best as you'd expect from any sim.
Loads of fixed wing opportunities as well but havent really had a go with them properly.