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Peter's answer is spot on if the OP's meaning of stack is one that turns the two "stacked" physical devices into one logical device. Peter's answer also applies if the two units are a VSS pair.
Besides the advantages already noted by Peter, MEC also often provides more aggregate bandwidth.
Also, MEC doesn't require LACP, but LACP would be a common implementation.
Lastly, if you activate Etherchannel, you may need to adjust the load balancing algorithm being used.
