My company operates both types, and both have their merits. Firstly, we do not use pallets in our A321's so loading is not an issue. In fact the a/c is lower which means that loading can be easier.
There was an addage amongst the 757 pilots " you earn it, we burn it!" The 75's fuel burn to the canaries was the same as our total fuel; ok they took 18 more passengers. I believe Qatar airways have ETOPS clearance foer their A321's.
There are still a lot of old polluting 75's around because they are cheap. Many companies will start looking at; if they are not already, replacing their 75's as carbon charges are introduced.
The 75 was a great aircraft in its hayday, but it is nnow getting a little bit un political in todays greener climate.