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Besides the additional bandwidth the stackwise+ ring port provides (dual 16 GBps duplex), stacked 3750s become logically one device which provides some redundancy features too. For example, you can define a Etherchannel using ports from multiple stack members and you get some NSF features. (Failure of a non-master stack member is like a chassis line card failure, failure of the stack master member is like chassis supervisor card failure.)
The 3750X series, when stacked, is also able to share power. Otherwise, except for the module card the 3750X supports, its much like the 3750E.
The original 3750 series offers much less internal fabric bandwidth, half the stack ring bandwidth, and "dumber" stack ring processing. (re: "dumber", Stackwise places all traffic on the stack ring, and the source both places the frame on the ring and removes it. Stackwise+ only places unicast frames on the stack ring that are non-local to the source stack member and the destination stack member removes the frame.)