If you mean the new 2010 models.. well, the Camaro is flashier, but the Mustang is more user friendly. And even though the V8 Camaro has over 100hp more than the Mustang GT, the Mustang is lighter and with the track pack it's only .1 or .2 seconds behind the Camaro in the 0-60 race. Car and Driver compared the Mustang, Camaro and Challenger, and the Mustang took 1st place. It's more refined, lighter, more efficient, and it sounds more like a muscle car than the other two.
I'd be happy with either one, but I gotta go with the Mustang. And, in 2011 the 5.0 will be back in the Mustang with 400hp.