Its worth asking yourself whether you will be using an external screen. If not then for the uses you mention a 13" is a really really small screen to work on. Kind of like holding a dinner dance in a cupboard, possible, but always cramped. 15" is also too small really for continual use. The 17" however can function as a complete desktop replacement no external monitor needed.
If you are going to use an external display its an easy choice. If not then its a hard decision.
ps
I had a 15" and found it just too small. I now have the 17" and its just perfect. It seems to be the magical size where bigger would be nice but not needed and smaller would start impinging on the work space). When I have no choice but to buy a new system if the 17" has not risen again I would buy a 15" + external monitor.
pps
The 6770 (17" MBP) is generally faster than the 5100(13" rmbp) for actual gaming
http://www.notebookcheck.net/AMD-Radeon-HD-6770M.43955.0.html
http://www.notebookcheck.net/Intel-Iris-Graphics-5100.91977.0.html