So here's the breakdown:
When I had a 15" Retina, I loved it. The weight was fine, the size was fine, everything about it was fine. It does all my day to day tasks perfectly fine. I had to sell it due to financial constraints but the one I had was the Early 2013 one with the 650M 1GB which is way better than Iris Pro. That being said, the Iris Pro is about 30-40% better than the Iris 5100. Combine that with quad-core and 8GB standard and you get quite a boost for the $400 difference.
However, this all depends on what you're going to play. If you're really only going to play League, I'll save you some time:
Neither the 15" or the 13" will be able to run LoL at maxed settings at Retina resolution without stuttering. The shadows are going to be what slows it down. So really, you won't notice much of a performance difference playing LoL on either the 15" or the 13". But with other apps? You'll definitely notice it.
These are the games you can play comfortably on the 13":
Borderlands 2 Low/Med
Half Life 2 series
League of Legends
DOTA 2 dialed down/medium (It runs sluggish even with 40+ FPS)
COD Black Ops
COD Ghosts on Low (Actually doesn't look that bad)
BF4 on a mixture of medium/low (Also doesn't look that bad)
D3 on Medium/High
Bioshock Infinite Low/Med
Can even play Skyrim on Low/Med
On the 15", the same games but one setting higher and some games become 100% playable: DOTA 2 becomes playable on High, Bioshock Infinite on High/Ultra, Borderlands 2 on High.
What else are you going to be doing with the machine? If you're going to be writing code and maybe rendering, you should consider the 15". It's a lot more "power built" and it's actually not THAT much heavier than the 13".
Also definitely get a keyboard cover. Water on keyboards is a DEATH sentence for MacBooks. Literally, good bye $1200 if you spill water on the keyboard, the logic board is dead.
And I love my Mac! Even as a PC gamer, my philosophy is:
I can play the games I want to play good enough to enjoy them but everything else I do with my laptop is WAY better because it's a Mac (At least for me). Productivity, word processing, editing, writing, even browsing the internet just feels and looks better.