Pros of repeating: a four year experience, more admission spots
Cons of repeating: cost, plus an extra year of high school, more competition for spots freshman year.
Personally, I'd rather take a gap year of travel or a PG year (where you can take a bunch of interesting electives or APs) than spend a second freshman year being at the bottom of the food chain.
Freshmen typically have more restrictions and less privileges than any other grade.
I'm not a fan of repeating a year already taken, but I know for many people it is a decision happily made. My advice is to consider what is right for YOUR child and go with your gut. People feel very strongly one way or another on this topic and some parents can get really, really obnoxious about what THEY think YOU should do, often based solely on a birthday. I KNOW Outliers says if you are born in January you will make a great hockey player and all that, but holding back isn't a one size fits all solution for every child born June or later. Consider the benefits and drawbacks, talk them over with your child and make an educated decision.