I don't think you should use the 300S's competition supremacy as a guide to whether it was necessarily a better rife than the 75 because FWB was competing with a fixed-barrel sidelever when the Giss Dianas were all break barrels.

Production of the 75 was delayed such that when it finally came onto the market the FWBs were already solid market leaders and Walther's LGR was ready to give FWB stiff competition. So Diana missed the boat, I think.