Utterly overwhelmed with Retina MBP options and need help.

I haven't purchased a piece of Mac hardware since my 2009 baseline Macbook Pro, which at nearly 5 years old is starting to show it's age something fierce. With (almost) all of the MBP models not being user upgradable, I'm a bit bewildered by the hardware options.

As someone who does a great deal of intensive photo editing in Lightroom and Photoshop, and occasional video editing in Premiere, the hardcore RAM upgrades are a no-brainer. However, the processor options on the new machines have me a bit confused. Would I be better off going with a high-end dual-core i7 13-inch model, or the base 15-inch models with Quad-core i7 but lower clock speed. Processor speeds with multiple cores and whatnot have always confused me even on the PC-building end of things, so any and all advice would be greatly appreciated.