TechSpot has overclocked the E6300 and E4300 and pitted them against each other and a stock E6700. The E6300's higher FSB seems to prevail, but the E4300's lower RAM requirement is definetely a bonus..budget-wise.
http://www.techspot.com/review/40-core2-e4300-vs-e6300-overclocking/
http://www.techspot.com/review/40-core2-e4300-vs-e6300-overclocking/