I've got one on a 300wm. 3.5-10'x40. Decent glass. Its held zero for the 100 or so rounds before the hunt, rode 1000 miles in the back of a truck, dialed to 350 yards to shoot a cow elk, and the 1000 mile ride home. Not sure how a 6.5-20 would be. I usually dont like that much magnification.
I’ve used the 3.5-10x50 CDS-ZL a bit. Glass is solid, feel comfortable hitting accurately for big game out to 500ish (or honestly farther), and like the simple illuminated duplex for a SFP scope that you can dial. Never truly tested zero retention so I can’t speak to that but didn’t notice any problems in general use. Have it for sale in the classifieds right now to try and fund some other gear if you’re interested haha.