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?
FMZ after looking at how you are using you system, aka with 10+ program open, in a development environment, you are going to more than likely have programs that can make use of all those cores, and even if the individual programs don't the sheer number of programs open will be better suited to a quad core. And if you come across flash content in a browser you will thank yourself for getting more cores in that case too, especially with a large number of programs open.
So since you are really going to be exercising your CPUs, taking a look at bench-marking results from Mac Geekbench results, etc will give you a good indication of how much total power you will have at hand. Going from dual core i7 to a quad core i7 is going to offer a noticeable improvement for you.
Oh and don't forget to get more RAM 8GB or more would suite you well.
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.
I own both a 13 macbook pro dual core i7 and a 15 macbook pro i7 quad core. I am an iOS programmer and I can say without any hesitation that the 15 macbook pro is almost twice as fast, especially if you are trying to run multiple apps at the same time. One recommendation is no matter what computer you go with, if you are planning to run that many applications at once I would get at least 8 GBs of memory.