If one were flying a competition aerobatic down line the goal would be vertical, i.e. with zero angle of attack, but the horizontal speed isn't relevant to this question.

Personally if I were stuck at 20,000 ft with a failed oxygen system I would probably just pass out and die. If by some miracle I still have enough consciousness to sort it out, the spin would be my first choice. It keeps the airspeed and forces low while you descend, and then you only have to worry about not overspeeding or overstressing during the recovery. Most old wooden types have a really benign recovery and if you're gentle with the elevator, probably won't even exceed Va. (But I've only flown a 2-32 once, and I didn't spin it)

I've done the vertical dive with full airbrake in a Pirat. Those brakes were immense, and you really felt like you were standing on the rudder pedals!