rmbp 13 vs 15

Hi!

I will be entering university later this year, and am in need of a new computer. Of all the PCs I've seen, only a few fit my likes (relatively thin, powerful and sleek), and rMBPs definitely tops my list. I need the computer to run some less graphic intensive game like dota2, and I love benchmark numbers, so pretty much all ultrabooks are out of it. Great battery life is also something I need (let down by the chromebook's battery).

so the question is?

should I pick 13 or 15

if I pick 13, I would get the i7 upgrade, if I pick 15, i would get the 4850hq upgrade, they are about the same price, but the storage on 15 is half of that on 13'

I like the 15' form factor though