MacBook Pro retina 13 inch or 15 inch?

Hello,

I've been thinking about switching from windows to Mac for a while,and in the end, I've decided to get a MacBook Pro with retina display. I would use this computer as a desktop since I have no need for portability other than the odd weekend trip or vacation.
I need the laptop for documents, many presentations, browsing the web, watching some movies, editing some photos and for a little gaming if I have the time for it.
Originally, I wanted the 15 inches because I like the bigger screen, but there is a big difference in prize between both laptops. Which one should I buy? Is it worth it to spend 400$ more for the 15 inch MacBook Pro with retina display?

Thank you for your advice.

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