Most glass shops will not guarantee the job of replacing just the window seal because old glass is brittle, and cracks easily. If they sell you a new window then a guarantee applies.

Another point is that there are differences in the quality of the seal itself. Buy an OEM seal if possible, and be sure its fresh rubber, that is more easliy installed than one that is old and might not have been stored properly.
That said, I have had seals replaced using aftermarket rubber that turned out OK, but the installer said for every 10 jobs like that one cracks.
The rear glass with defroster wires is more expensive too, FWIW.
Dieseldiehard