Even in CPU-intensive tasks, the difference is going to be more of the "noticeable in synthetic benchmarks" variety, rather than "I see it in the real world" variety. I'd save the money and put it first towards an SSD option, then more RAM.
Processor speed is almost flat these days, so minor bumps aren't worth the money.
My very aging MacPro got a new lease on life when I replaced the boot disk with an SSD earlier this year. There's nothing better for improving perceived responsiveness.
One way to accomplish this would be to test the performance in a virtual machine configured to represent a more limited processor.
Assuming you're using a Mac which came with a pre-installed OS, you'll probably need to buy an additional Mac OS license, but the virtualization can be done for free with VirtualBox, or for a moderate cost with Parallels or VMWare Fusion.
In the case of VirtualBox, when you create a virtual machine, you can adjust the amount of RAM, the number of CPUs/cores, and the Execution Cap to limit the virtual CPU to a percentage of a real CPU. I believe Parallels and Fusion offer similar functionality.
Most virtualization use cases for Mac involve running a virtualized Windows OS, so you will find a lot more information about Windows licensing with these products, but virtual Mac OS is a legitimate option, as long as you comply with Apple's licensing requirements.
Apple allows virtualization of many of their operating systems as long as the hardware platform used is an Apple one, but the specific details vary depending on the operating system version and how you got it.
For example, if you buy a Mac with Yosemite pre-installed, you don't get virtualization rights for that OS, but if you get your Yosemite license from the App Store, you are granted the right to run two additional virtual instances of Yosemite on your Mac, as long as it's for "personal" or "development" use.
Because there are different rules for different versions, the best way to ensure you're complying with Apple's legal requirements for virtualization would be to contact Apple directly and explain your requirements.
Best Answer
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.