In 1980, a lighter, more economical 3.8-liter (229-cid) V6 rated at 115-hp replaced the old familiar 250-cid I-6 as the standard engine. The new v6 Camaros now come with 200hp stock.
The 2008-09 v6 Mustangs have 210 HP. In 2003, they were rated at 190 HP.
Of course, horsepower ratings are only one specification, curb weight of a 2003 Mustang
was 3000 lbs. The new Camaro's curb weight is 3700 lbs. The earlier models were approximately 3300 lbs.
All in all, I'd say that it's going to be a toss up. The first person to start adding performance mods will get the advantage.