Headset for both plane and heli: buy for plane, use adapter for heli, or vice versa?
There's no issues with the adapter either way, as long as it's good quality. The helicopter versions are coiled for a good reason: to keep it out of your way, so I would buy the helicopter version. In most fixed wing airplanes the coil will be absolutely fine, if there's not enough reach you can get extenders pretty easily.