This is interesting question. I thought about this too. I would probably try to make the "SOS" because (like Andrew S has mentioned) it would be clear to more people as opposed to "V" or "X".
I would make the "V" symbol if I knew SAR people are looking for me or if I had no time or energy for making the "SOS" symbol.

Similar question arises: what would you use for acoustical signaling (whistle) or light signaling (flashlight)?

..._ _ _... (SOS) or _ _ _ (3x sound/flash) ?

If you were in Alpine area would you use the Alpine distress signal(6 signals in one minute, then one minute pause, then repeat) or rather something else?