I believe the main difference between them is that the b757 is out of production. I don’t know if there are too many spare parts in the market today, but there are speculative companies specialized on buying big amounts of spare parts to raise the price. The FM computers are old and slow. The A321 use cargo pallets. It doesn’t mean that manual load of the cargo is less efficient than pallets system, but need special machinery different from the rest of airplanes and that means money to a company. B757 may fly ETOPS, the new BA company 757 all business fly that. I have heard from the engineers that flights less than 2 and a half hours the A321 is cheaper in terms of petrol than the B757. Further that time B757 is more efficient, but I have flown the B757 in 1 hour flights and found it is more a question to get the clearance to climb to the optimal, usually 370, 380 or 390.Flying the 757 is easy when you get used to the beast. It permits you almost everything. You can land and TO in very short rwys and I have landed on airports where the A321 were missing the app due to the turbulence. The B757 cockpit is big, ergonomic and very well ventilated.