I love spair air. I think that the diving world would be a safer place if every diver carried them. For longer deco dives, spair air can be worn on bandaliers. Remember, green spair airs are for your deco!
I use a 30 CF slung bottle when I dive singles. According to Frank's calculations with my sac rate plugged in, after ascent and a 5 minute safety stop, I'll surface with about 34 CF of air left in my bailout bottle.
PS
Unless you have the absolute need for doubled 120s (or are clinically insane as I am with my LP125s), don't bother. Those tanks are more than anyone would want to deal with climing up a rocking boat ladder in 6 foot swells.