E6600 = 9x multi, E6750 - 8x multi.
E6600 = higher temp per clock, in other words, an E6600 @ 3.5 will run hotter (and at higher volts) than an E6750 @ 3.5.
Unless you're using an older board that has really restrained FSB, or you've got some ridiculous ram that you refuse to downclock, take the E6750.