I guess the 752 is used in the quieter parts of the summer and larger aircraft in winter when they only have 3 NYC flights a day to compete with as opposed to 6 in the summer, plus larger aircraft on PHL.
The DL B764s are used on LHR etc so I'm guessing their cabins are amongst the nicer ones they have. The 763s they used on ATL in the winter tended to have older cabins, no ptvs etc.