What should I do?

Going solely on the screen size, I'd say you have to go 15". When it comes to gaming and editing of any kind, you're going to want that extra screen size. If you plan on using a different monitor then it's a different story. Also, is HD size an issue for you? Externals are so big and cheap now, there's always that route if so...

 

The only downside of the 15" is the weight. Other than that, it is a powerhouse, and I would recommend it for any heavy work as long as you can bear the 4.5 pounds. I would look into getting a 2012 used model though. The baseline 2012/2014 models are very similar power wise, for both the GPU and CPU. The 650m is actually 7% faster for 3D graphics (notebookcheck.com), and the Ivybridge processor in the 2012 model is 11% slower than the new 2014 model (geek bench for mac). You can find them for as little as $1200. They are exactly the same externally. Just something to think about.

 

I also vote for the 15". You can supplement the storage, but not the screen size (besides an external monitor). Also, the screen seems extra small on a 13" rMBP to me; perhaps because of the high resolution. I think you'll appreciate the real estate for the uses you listed.

 

Another reassuring vote for the 15" model.

 

Mathematically speaking, 15 > 13.