Since you are already in 4h and are only planning a few meat birds, definitely go through the auction! First and foremost, it is the experience that matters.
Second, you are more likely to get more money for a limited number of birds that way.
Profit is higher when selling open market when you are working with larger quantities. Feed in cheaper in bulk if you are feeding 100 birds. Less is spent on infrastructure (pens, etc,) per bird equals higher margins.
People attending 4h auctions are there for the chance to further projects and encourage youth. People buying on the open market are looking for the best price.
Around here, open market, free range CX are selling around $2-3/ pound dressed. 4h auction, prices are unlimited.