Although Dodge was marketed slightly upscale of Plymouth, the differences were minor iMO. More like what’s your choice, a Buick or an Oldsmobile? And not so much like do you want a Chevy or a Buick? In some cases I thought Plymouth interior styling was a little sharper than equivalent Dodge models - the 64 Sport Fury and 67 Satellite/ GTX my own examples. Of course their were exceptions, the Dodge Charger models one obvious one and probably the Challenger vs the Barracuda. Of course styling is subjective and everyone has an opinion. In the latter half of the 60s, Dodge no doubt poured on a big performance campaign with Dodge Fever and the Scat Pack and Sheriff Joe Higgins. Plymouths was quite a bit more subdued and mainstream with the Plymouth is out to win you over campaign, although the ads for the introduction of the 67 GTX and the 68 Roadrunner were performance oriented. I think marketing forces lead Plymouth to concentrate on its budget Musclecar, the Roadrunner, while Dodge went more upscale with their Charger.