Functionally speaking, where will I see a difference between dual- and quad-core i7s? I am ready to buy a new MacBook Pro. I am drawn to the portability of the 13" and the better specs of the 15". If you take money out of the picture I would really like to know where I would see the difference in performance between: the 13" i7 Duo Core 2.9GHz and 15" i7 Quad Core 2.3 or 2.6 GHz?
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.