There are so many nuances to clock speeds. Use this as a basic guideline...
http://www.cpubenchmark.net/high_end_cpus.html
Sometimes, two CPUs will have the same clock speed, but if one is a 22nm and the other is 32nm -- clock for clock the smaller will be faster.
Or some AMD CPUs which appear to have a higher clock speed than their Intel counterpart, are actually not faster.
Generally, I'd say don't get a Xeon unless you need to build a dual CPU rig.