Thicker paint tends to hold to the roller better and unloads more solids to the surface being coated. Thick is good for ceiling paint which usually has additives to enhance flatness. The thicker paint will hold a wet edge better and show fewer lap marks. As a general rule, I would agree with the salesman in this case. The whole idea is why put on two coats if one will do. The better paint is more likely to save you money even though it is more expensive per gallon.