It's all about operations per second. I7 baby.
Let me actually give me some facts rather than those above me who's answers are comparable to state propaganda of North Korea.
Intel's naming scheme works like this:
i3 - 2 physical cores
i5 - 4 physical cores
17 - 4 physical cores + for every 1 core, there are 2 cores to spread load between
Basically, in short, the i7 has 8 cores to balance load unto.
Whether you need this? Are you a multitasker: someone who wants to do multiple CPU intensive tasks at once without a hit on performance (or a minimal one as such) on each of those tasks?
Or do you want loads of power for one thing?
a 4 core processor will handle multitasking fairly well. but not heavy multitasking.
In an i5, you can have tons of chrome tabs open while watching youtube.
In an i7, you can play games at full performance while rendering videos.
If you want to do a single cpu-hogging thing at once, get an i5. If otherwise, get an i7.