My vote goes to 757 (of course I am a pilot, not an accountant)
Accountants love the 321 coz its... cheap... so it's good if you're just looking at the cost front... However, revenue is where you get your money. 321 has "unbeatable costs", but the 757's revenue gaining capability is on a different league to the 321... ie: it depends where you want to fly those planes to.
Until (maybe already) the margins are eroded completely by the fuel prices, the 757 is still the choice. And finance managers love it, but the accountants hate it.
If only the 757 flies where I am, heaps of possibilities that the 321 couldn't match. But then, the guys giving out the airport expansion tenders are having a nice time because the 757 never made it here.
PK-KAR