Apples and Pears
Interesting discussion. Reality: comparing apples and pears
B738 v A320? try getting a cargo container in a B738....
Reason A320 P2F is basically scrapped? No available airframes as the demand is so high. B737 lacks volume.
A300P2F: DHL has bought airframes which are @ 30000FC/HR which are presently undergoing conversion. The aircraft fits profile for OPS by DHL/FEDEX and is cheaper to run as B744.
Reason some aircraft get converted more than others is simple, good payload/volume and purchase price. Availibilty depends on numbers built and operating cost.
I've been working on the maintenance side for 20 odd years, even worked for a guy who started 2 airlines, the first with a all boeing fleet, the second with all airbus. My preference? An aircraft with plenty of work, I don't care what it says on the data plate. One is as bad as the other, but in the end they all fly after enough money has been spend on maintenance(more money, less MEL's)