How can you work out which CPU is faster or more powerful?

there's no simple answer, generally what you get is a "CPU score" by putting in various benchmarks. This is just because there's an instruction set with each CPU that make it faster in some things (takes less instructions) or slower (takes more instructions).

the i7's are more recent and will consume less power and without googling, is probably the better choice of the two.

There are various tables published, including by Intel themselves:
http://ark.intel.com/
or
http://www.cpubenchmark.net/

BUT you need to know the actual model numbers as well.

Thanks guys, appreciated.

The price will normally provide a clue!
My pc has an i7 processor with 8 cores. When I'm converting video from one format to another for a number of files, you can see all 8 cores in action - processing 8 videos at once

Displayed in task manager