The longest route I have flown on the 757 was a non-stop flight from Orlando to Manchester, with a full passenger load on an Air 2000 757 in 1993. We also has full fuel tanks but it's wasn't really much of a struggle for the plane. The flight time was 7h 41min but we had 9 Tonnes of fuel left on landing out of the 35T we took off with, so plenty of endurance left
AA don't fly the 757 into MAN anymore. Continental killed their JFK route, both airlines flew it at the same time with 757s (CO to EWR) AA now don't fly NYC-MAN at all and Continental now fly a daily 777, so no prizes for guessing who won that battle!
Also, several airlines fly the A321 and B757. Air 2000 fly 14 757s plus 5 A321s with another 3 A321s on order. Airtours fly 6 B757s with 2 A321s and another 2 on order. Monarch fly 7 B757s and 3 A321s. While they may have similar capacities, the 757 is a much more versatile and dynamic aircraft for the charter airlines. Also, It is about 15-20 Tonnes lighter, making it cheaper to operate on routes in Europe. Also, sharing a flighdeck with the A320 allows airlines to swap capacity between seasonal bases. Formerly a base that could support a 757 in summer would only need an A320 in the winter, needing two separate sets of crews. Now an airline can have A321s in Summer and A320s in Winter, all with the same crews, maintenence personel and spares. I hope this explains why some airlines operate both the 757 and the A321, even though they are very similar, they have very different roles. Of course, the B757 is compatible with the B767s used by Air 2000 and Airtours at their major bases and so they have advantages there over the A321.