Unless you are a significant power user, or deal mainly with programs that eat a lot of CPU power (which I don't), you most likely won't see any noticeable difference.
Whether it's worth the extra $100 is another question entirely. As noted, even though I'm not a power user and most likely would never notice the difference, I went ahead and upgraded to the Core i7, and I did so for a few reasons:
1) The upgrade is only $100. $100 is nothing to sneeze at, but in the grand scheme of thinigs, wasn't prohibitively expensive.
2) I wanted to future proof the computer as best I could, and I felt this was one way of accomplishing that, and as noted above, all for $100.
3) I thought it would help with future resale value. My thinking was that 2-3 years down the road if I want to sell and upgrade, a core i7 would be more desireable from a re-sell standpoint than an i5.