if you are going support cho, I don't recommend running anything offensive in the tree. I mean if you go 21/9 you are playing as if you are going to be a carry, and you flat out won't get gold to benefit much from it. You're dmg early on is ok, but not good enough to waste so many points into the offensive tree. 21 points in defensive is not bad as it will make you as tanky as possible. the buffs to the defensive tree are quite good, and even this alone helps make you decently tanky without any items. With cho's natural health from ult, for a support you offer some utility and a decent amount of tankiness.
I still feel 21 points utility is always the way to go. the roaming movement speed is not to be understated, the gold income is pretty huge you get, as well as the mana regen you get for going so deep in the tree. It's like a mini athenes and is stronger than you think. and putting 9 points in defense is actually still decent as opposed to 9 points offense is useless on cho.
Still feel there are better supports then cho, but if you are intent on playing him support I think 0/9/21 is the way to go with 0/21/9 as a viable option.