Maybe you are just be lumping A319's and A320's together, but for the record the A319's were not that old at time of merger and were still in pretty good shape. I suspect you are thinking of the A320's which are older and were definitely showing their age. First A320's were delivered to NW in 1989 while the A319's deliveries did not begin until a decade later in 1999 with final A319 delivered in 2003.
The experience was about the same honestly. Barebones with cloth seats, perhaps the walls were more yellow on the A320. Shortly after the merger, I was on a DL flight and was wowed by the AVOD on an exSong 752. It’s been a while since I checked the DL fleet (no time) but I remember that there were A320 frames delivered post-9/11.