As already mentioned, the major difference is the range. The B757 is better suited for medium haul routes, the A321 on short routes with the same pax. That's probably also one reason, why the A321 isn't a hot seller in the US - not even amongst big Airbus operators, as UAL - but it is in Europe. A big part of the major inner European routes can be flown with the A321, AFAIK.
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