Got the 2010 Mustang GT Prem.
1. It is the better looking car in my opinion. Both inside and out. Now if it was the '05 model against the camaro it'd be a different story. The interior in the Camaro is way too cheap.
2. The front of the Mustang is way more appealing. The Camaro is lacking something, I think it's that really plastic looking grille. (I remember thinking the same thing about the '05 mustang having a really cheap grille)
3. More opportunities on the Mustang. Roush - need I say more?
4. Interior is like something of a cadillac, lexus or bmw. Its so nice.
5. Ambient lighting is done way more discreetly. It impresses but doesn't look like an alien spaceship.
However, don't let the cheaper price fool you - they aren't much of a difference.
2010 Mustang & 2010 Camaro both are some of the coolest cars. I was the first one to own a 2010 Mustang GT in my state, would love to roll around with a 2010 Camaro sometime. (Saw someone driving one today, though I think it was a v6)