if the same car was in both premises then it would come down to reputation
the premises you describe is exactly the same as a car sales near me, old petrol station forecourt etc and the guy is now in prison for clocking cars, this is more likely to occur with a small independent trader
BUT if the place had a good rep with a knowledgable salesman then i would rather give him my money than a faceless upmarket Co
it really depends on that initial contact, i have had poor experiences with both - a small trader was not interested in the problems i found with the car and gave me the impression "if you dont buy it someone will !!" and at arnold clark i was buying a brand new clio and a Z off them (best part of 30k) and the guy wouldn't let me leave to consider his offer unless i left him a credit card to ensure i returned !! i walked away from both