First of all you can pretty much have a look at the differences on Intel's website:
http://ark.intel.com/es/compare/88188,88196
The i7 has integrated options available
The i7 has hyperthreading
The i7 has 2 extra MB of L3 cache
The i7 runs 100mhz faster regardless of the # of active cores (3.7 with 4 core, 3.8 with 3C, 3.9 with 2C and 4 with 1C compared to 3.6, 3.7, 3.8 and 3.9 ghz on the i5)
So basically it depends of what you are going to be doing with your PC
If you plan to use adobe premier, vegas, or any multithreaded aplication for 3D modeling, editing, encoding, etc go for the i7 due to those tasks DO benefit from hyperthreading and the extra cache
Gaming wise, the i5 performs roughly the same as an i7 then get and i5 which is cheaper and invest more on your GPU. Games DO NOT benefit much from hyperthreading and the differences in clock speeds are minimal
Hope to have answered your question
So it leads us to say that it depends on the tasks you're going to be performing
If you plan to play games just get the i5 if you plan to do any task regarding content creation then go for i7