Functionally speaking, where will I see a difference between dual- and quad-core i7s?
The analogy is the choice between having two really fast employees, or having four quick ones. If not a lot is going on, the two fast ones will be tearing things up. But if things are busy, and a lot is going on, having twice as many will really show up.

Note that with the i7 Quad Core, if you need a lot of speed from one or two cores, you can get it through the Turbo Boost. 

I would lean toward as many cores as possible, because I like to have lots of programs open and be switching in and out. If you like to work on one thing at a time, and if those things aren't all that compute intensive, it may be a harder decision.