I recently asked a similar question, regarding whether the B's or C's have generally shown themselves to be better overclockers. The answer was that they are both similar.
I bought a 1Ghz AXIA Y last week with the 266 bus. It happily clocked to 1450 with air cooling, and runs 1333 at default voltage. For obvious reasons, I have no reservations about C chips being less overclockable.
Essentially B and C processors are exactly the same CPU with different default multipliers. If you are willing to alter the L6 bridges to set the default multiplier, it makes no difference. Otherwise, to avoid problems with some KT133A motherboards, go with a 266 to get the lower multiplier from the factory.