Nope, a 955 at 965 speed is as fast as an original 965 (there are benchmarks on this). It only depends on budget, both CPUs are pretty much the same.
The 965 has a default vcore of 1.4, while the 955's vcore is 1.35, which means that the 965 doesn't automatically OC higher.
But in the end it only depends on if you can get your hands on a good chip and if you do, 3.7-3.9GhZ is pretty much the max anyway.