Helicopters tend to be better for not making you airsick (in my experience - and I've done a lot of heli and plane photography), unless you're really getting thrown around. The act of peering through a viewfinder can upset your inner ear balance and kick off sickness (as in small children travelling in cars and cannot see out of the windows). If you are prone to sickness, and even if you aren't, I'd recommend keeping your non-viewfinder eye open and use it to passively look at the ground and horizon while taking your photos. Your internal balance mechanism then has a reference.
300 Lens? A prime 300 or a zoom up to 300? A 300 prime lens will be too long, unless you want relatively close up pictures. However, the movement and vibration will limit you to high shutter speeds and ISO values. Almost all of my photos are taken with a 24-105 and most of them are in the 50 - 80 region with only a few going up to the 105 end. (on a full frame camera - I'm too tired to think what that is on a crop sensor). Too wide and you'll capture parts of the helicopter like landing skids maybe, and blades, and also more reflections from the windows. Wear dark clothing. Keep the lens close to the window, using your hand/fingers to keep it from touching. Try to use a speed of around 750 min, and (but less critically) an aperture of f8 if you can, so ISO as required. You can fix noisey pics but not blurred.
And take a bit of time off now and again from taking photos to take in the view and experience.
