Not sure about brake / shifter clamp zone, but the standard stem/handlebar clamp diameter is 25.4mm. OS is 31.8mm. OS is becoming pretty standard.

I like OS just because it looks flippin RAWK! With a good bar and stem, you'd be hard pressed to actually "feel" a difference unless you're senses are acute and you're a monster. I think 25.4mm is plenty adequate, but I'll probably never willingly choose it over 31.8mm because OS looks flippin RAWK!