The C core has more multiplier headroom, which most likely means more potential. 1200 is already pretty high, so you'll have to take either fore in .5 multiplier steps and 1Mhz FSB steps either way. Telling the B core to go from 12 * 100 to 12 * 133 is pretty drastic, so you'd have to lower the multiplier a lot, that's 12 Mhz for every 1 FSB increase, as opposed to 9 Mhz for the C core.
Either way, have really good memory to be able to handle the high FSB frequency, plus a good overclocker like the Iwill-KK266 would be your best bet.