I'd definitely go for start + select.
The power switch I don't like for this function. Everytime I have to use it, even for turning volume, I have to firmly grasp the gcw0 with my left hand, and put force with right on the switch, few times I was almost lost the GCW0 from my hands, almost fell on the floor.
The L+R and others, are main buttons in games. I mean, imagine I am playing let's say metal slug, and I want to go to the menu to change an option or save state. By pressing all these buttons, I also told the game to throw grenades, I lost some of my grenades.
But with start + select, the worst thing that usually happens is that I accidentally paused/unpaused the game.
I think it's the best option.