Is has been my experiences that on Airbus 319/320 or any of the DC-9 series, I have been offered pre departure drinks, and on any of the 757-200 or 300 I have not been in the last two months.
Pre-Departure Drinks offered:
BWI-DTW DC-9
FNT-LAS Airbus 319
LAS-FNT Airbus 319
LAX-LAS Airbus 319
LAS-LAX Airbus 319
DTW-PHL Airbus 320
No Pre-Departure Drinks offered (including water):
LAS-DTW 757
PHL-DTW 757
DTW-BWI 757
DTW-MEM 757
Not sure if this has anything to do with it, but 4 times on a 757 no pre-departure drink, and all other aircraft, yes.