Do I need Macbook Pro Retina 15' or 13' will do?

First of all I'm not gonna buy anything until the MBP retina is updated to Intel HD 5000 and Haswell Chip, but the thing is I'm not sure if I really need a quad core either ways? I prefer the 13' size a lot more but if I need a quad core I would probably buy the 15'...

First I'm a web designer so I use Photoshop a lot and sometimes I work with very high resolution...
2nd Im planning to install parallel and use in it Linux and Windows, for Windows it will be very light using, ill just use it to run couple of softwares that doesn't work on Mac, as for Linux I'm still learning about it and so far all I use are programs built in it and just basic use.
And I might use it for gaming from time to another.

I also want to know whether the quad core make installing programs faster or it has nothing to do with it?

And I would appreciate it if someone described to me the basic difference between a dual core and quad core cuz I've always went with 13' laptops so I never owned a quad core to be able to tell the difference.

PS. I can afford buying 13' retina with maximum customizations (3.0GHz Dual-core Intel Core i7, Turbo Boost up to 3.7GHz) or buy 15-inch: 2.7GHz
with Retina display.

Mac

Follow this Question