I don't know if it will help if your comparing a something like a laptop with http://ark.intel.com/products/50072/Intel-Core-i5-2540M-Processor-3M-Cache-up-to-3_30-GHz to one with http://ark.intel.com/products/52231/Intel-Core-i7-2620M-Processor-4M-Cache-up-to-3_40-GHz?
Or similar gap for a desktop.
I think for most people the i5 is going to be a better choice, if you want or need an i7 then by all means get one.
This is something I agonized over for at least 3-4 weeks before getting my newest computer.
I am not a bleeding edge gamer, I don't do video editing, and while I will probably do some video editing sooner or later, it will only be occasionally, not every day.
Based on that, I couldn't think of a situation were I personally would be limited by a fast i5 and not with an i7 CPU.
So I decided to go with i5 quad core that runs at 3.1 GHz, a good but not great GPU, and only 4 GB of RAM.
Except for the RAM, which I am planning on upgrading to 16 GB on my birthday, this has worked out very well
I am using more than 90% of my RAM on a daily basis, so going to upgrade it to 16 GB so I don't have to worry about it.