I saw your post in another users thread about being on the fence about the same decision and decided to do the ol' copy/pasta on yours. I hope this helps you out.
In terms of games (and other graphically intensive tasks), it depends on what you are referring to specifically. The Iris Pro can handle some older, yet still modern games but for anything else you're going to want the R9 M370X, which will still struggle on AAA titles. It is a mid ranged dedicated GPU. Use the search function on
NotebookCheck in order to see benchmarks for specific titles on each of these GPUs. Just select the magnifying glass image on the menu bar and type in the name of each GPU separately, and the first link of the results provides the statistics. This will allow you to gauge which GPU you need. It is wise to keep in mind too, that the M370X is only offered in the higher end 15''.
Once you have the GPU selected (use NotebookCheck) the rest is negligible. .3GHz clock speed and double to storage isn't worth the price difference, in my opinion, if you won't be utilizing the dedicated GPU.
Unless you don't want to play a specific game that shows poor results on the benchmarks of the Iris Pro, I would grab the baseline model. It is more than enough to handle photo and video editing.