Plane every time for me. Got little un a half decent car for xmas, and it’s fun for a little while but gets boring quickly. Far more to do with a plane. Mind you I haven’t flown in ages. Given a smallish electric plane no need for anything particularly special as a flying site. Personally I far prefer planes over helis, though I can fly both (to a basic standard).
Mind you if you can fly planes, don’t you already own one, and why are you asking our advice?
FWIW my son is just coming up to 7 and just about up to driving the car (a 1/18 4×4 buggy) – 4yo has had the controls but only with my hands also on them.