I would take the A/C, especially on a 4-speed car. Stick cars with a/c are extremely rare because most of the manufacturers prohibited it from being selected on the higher performing engines. No a/c was available on any 426 Hemi or any 440-6bbl car, but you could get a/c on a 440 Magnum 4bbl, even with a 4-speed (very rare). When I was hunting for my 72 455-HO T/A, I especially wanted a 4 speed with a/c as it is the rarer of the option combinations. There is nothing like cruising into a car show in 100 degree August heat with the windows rolled up and 40 degree air blowing through my long blonde hair.....sorry I thought I was in a York Peppermint Patty commercial there for a second.
I think a/c would add to the value of the car. But remember there are a lot of obscure little (read: expensive) parts that have to function perfectly for the system to run right. Just run an ebay search on the term "POA Valve" and you'll see what crazy prices people pay for an NOS valve for the GTO/Corvette/Camaro crowd.