They are both excellent cars, both should be roughly the same performance and fuel wise. Test drive them both and decide which you like best. There are features and options that the mustang has that the Camaro does not. ( sync, glass roof ) Also check out insurance on both, sometimes it can be quite a bit higher on one than the other. Maintenance items should also be considered too, tires can be quite expensive if they are a unique size.
My personal preference would be for the Ford, I like the interior better.
The Chevrolet Camaro is a great vehicle! Have you had a chance to test drive one yet? I wanted to point out that the Chevrolet Powertrain warranty is five years or 100,000 miles and Ford's Powertrain warranty is only five years or 60,000 miles.
Having driven both while my wife was car shopping I have to say the Mustang is a far superior car That's why we got the mustang also the trunk on the Camaro is allot smaller especially in the convertibles.