The E-175 isn't an RJ, but scope allows a carrier with RJs to fly it for mainline. Some passengers prefer them to a 737, no middle seat and many of the same amenities. Assuming that most passengers understand the intricacies of the mainline/regional relationship is probably false.

Ticket price, past experience with an airline, ticket price, reliability and ticket prices are the factors I believe... Although two of them may not be actual factors to the average consumer