With the exception of the rear spoiler, I think the Camaro looks uniformly better than the Mustang. And, I'll forward my opinion as unbiased since I've never owned versions of either car!
Both the exterior and interior design of the Mustang just looks "tired" to me. Once the excitement dies down, maybe in 2-3 years, I have the sense that people will just tire of the yet again "retro" look of the Mustang. The Camaro is edgy, forward-looking (inside and outside); a proper re-imagining of what an American car with a storied muscle car tradition should stand for in the modern era. Ultimately, I think this is the sort of thinking that will bring new and younger buyers to the line.