To verify the difference in general speed you should head over to geekbench and compare the scores.
However, to answer your question, we should ask you a question. What are you using your computer for?
I have an i7 iMac and I have a little utility called MenuMeters that shows CPU utilization. 90% of the time I am only hitting 5% to 10% across the 8 cores. But if I run Handbrake, they all ping out and it flies through an encode in no time.
But I only do that once in a while.
It's kind of like saying should I get a Lexus or a Ferrari? If you are only going to drive on city streets, a Lexus is fine because you couldn't take advantage of a Ferrari's speed on city streets. But if you plan to race on a track, then the Ferrari will kick the crap out of the Lexus.
But some people can afford a Ferrari and don't care that they never really use its power. Other people prefer a Lexus since it is cheaper and has more power than they typically will need.
So, i5 will get most normal computer use done without breaking a sweat. But if you are going to hammer your processor with serious work, get the i7.
I batch encode a few dozen video files most weeks. And also need to render effects/ color correction while editing. Which takes a while for my Mid 2010 i3 iMac to take care of. So definitely the i7. Which is what I was planning in the first place, I was just curious how the Adobe suite handled it.