What affects the speed of a computer more: Cores or CPU Speed?

Perhaps a bit of a luddite question, but I am in the market of purchasing a new computer. Many computers have roughly the same speed in terms of gigahertz (ghz), but have different multicore processors. For instance, I am seeing 8 core 3.1 ghz machines versus 4 core 3.5 ghz machines. I was wondering how this actually translates in terms of daily activity. What does the core type affect and how is it different from the cpu speed?