Like others have said, Tirerack is a reputable company and what they recommend will work.
Don't worry about the less performance characteristics of any snow tire because that's not what its purpose is. All you care about is being able to move in the snow vs. the stock KDW's that will will leave you stuck in 2 inches of snow. People who buy snow tires do so because they have to get to work (and the mall and the grocery store etc). They give up some of the dry grip just because they know they have control in the snow.
This shop sounds like they are giving you a line to not get the sale, but also get you into more expensive, bigger tires. There are no snow tires in the stock 215/50R16 size. So, this store would have to sell you a different sized tire which are "improper" by their own account.
Difference in sizes besides price? a skinnier tire cuts through the snow like an ice skate with control. A wider tire will ride on top of the snow more like snow shoes. This is another reason people buy smaller width tires.
