If you go the Chevrolet Website and do a competitive comparison of the 2011 Camaro SS and the 2011 Mustang 5.0 GT you can see a side by side comparison of each of their specifications. If speed is what you are really looking for the 2012 Camaro ZL1 is said to go 0-60 mph in 3.9 seconds.
They're pretty even. But I'd go for the 5.0 mustang GT. It's cheaper ($29k vs $31k), has a faster 0-60 time (4.8 s vs 5.0 s), gets better fuel economy & has a good reliability rating on Consumer Reports.