There are less Buick collectors around but that is tempered by less Buicks vs Chevys. But the GS is the better car imo since it's far scarcer than the Chevelle SS. In the end it comes down to condition. Consider rust free orig sheet metal or replaced body panels or patched areas. That's a big swing . Original stamped VIN on engine and Trans are big pluses as are original data plates, factory paperwork, build sheets, etc. Color and options play a strong role too. The right color, the right docs, matching #'s, orig sheet metal, 1-3 previous owners are all things that drive the value. 2 seemingly "identical" cars could be 2X difference in price because of all "the extras."
Buick GS stage 1 is pretty good bang for the buck. Probably one of the best underrated muscle cars out there. Everyone knows Chevelle SS, Boss Mustang, GTO's, Hemi Cuda's and Charger 6 packs, Not so many are familiar with Olds 442 and Buick GS. The GS is even less widely known than the 442. And the GS455 over the GS400 is a nice step UP. My grandma owned a 1964 Buick Wildcat 4 door with the 400 ci engine. That was a pretty fast car for what it was. Had fun driving that in the early to mid 70's.