the answer is even more simple than that!
Intel obseletes its line-up on a 6-month basis. just as the E6320 made the E6300 obselete (same price, more cache) this E6750 makes the E6700 and E6600 obselete.
in the E6xxx section you should really only look at E6550, E6750 and E6850.
i bought an E6550 only to later regret i didnt add 20$ for an E6750. the price gap between E6750 and E6850 , IMO, isnt worth it. an E6750 is the best price/performance right now (maybe after a Q6600 but thats a different price range).
so go buy an E6750 and be happy!