Time to display my filthy secondary knowledge:
Each has only been used once, and not under identical conditions. What we do know is that GRC is forbidden to use in Las Noches while CO is not. However, when CO is used, it wrecks much of the dome over Las Noches -- the same effect that CO is banned to prevent, though that is referred to as being total destruction. Furthermore, the one use of GRC wasn't actually that impressive, while the one use of CO wrecked Ichigo's shit and broke his mask completely. There's also a translation issue, IIRC Ulquiorra called CO "The strongest Cero in our released form." That implies that, while in an Arrancar's released state, CO is either the strongest form of the ordinary Cero, or the strongest variant of the Cero, which have radically different meanings. Furthermore, we have only two samples -- we've never seen GRC used in someone's released form, and we're relying solely on two datapoints and a vague line of dialogue to discuss power levels in a shonen series to try to determine which of the big glowy special effects energy blasts is more powerful.
tl;dr: Nobody fucking knows.
Addendum: I'm ashamed to know this much about a series I never watched or read past the first arc because of the sheer volume of crap available for media osmosis.