OEM is always the best choice, if you can find them. The biggest problem with G-body doors was the seams. Not so much the outside, but the insides. They didn't do much if anything for moisture control, and when the bottoms of the outer skin starts corroding, it also was corroding the inner section of the door shell as well from the inside out. Total PITA to fix properly. Door skins are great if you can find them as long as your metal on the inner shell is in good repair. Some aftermarket is better than others.
If you get new door shells, or do any repairs to your originals, it may be wise to seal the door seams on the inside with liquid sealer after fixing and painting everything, then put all your door guts together. See Donovan's build thread. He's using cavity wax specifically for this purpose.