wrote:

Since most aircraft don't have oxygen, the question is rather pointless.

If an aircraft does have oxygen, it most likely has the capability
of easily clearing mountains and the choice is obvious.

A portable oxygen system is quite affordable (and actually reduces
fatigue during long flights even if you don't fly at altitudes legally
requiring to use it); quite a few normally aspirated piston singles
can fly quite happily at these altitudes (well, not by much, but
they can)

--Sylvain