I am not sure what you mean with task specific performance, so you will get a slightly generic answer.
There are a lot of different metrics for comparing CPU's, and it depends a lot on what you intend to do with them. The most common meta statistics are:
And the ever popular weighted index such as
Performance in turn can be measured in many ways:
You should compare the CPU performance against the intended application. If your application is a virtualization server with 200 virtual hosts you will want a very different CPU from what you need in a mobile phone or in a firewall. Outside the HPC world it's rare that a single test captures most of the performance characteristics you are interested in.
One thing you can look at as well is to look at the instruction sets included on the chip. Does the chip support extensions like SSE2, SSE3, SSSE4, SSSE4.1, SSSE4.2 extensions as well? These will effect task specific performance of various CPU's.
http://www.cpuid.com/softwares/cpu-z.html --> tool for checking processor instruction sets.
http://en.wikipedia.org/wiki/SSE4 -> Streaming SIMD Extensions 4