A greater graphics capability gives developers more to play with. The Xbox One has less than the PS4, meaning if developers want to create graphically intensive games they will make them exclusive to the PS4 leading to the PS4 having the best looking and graphically intensive games. The CPUs are rumoured to be similar so whatever the One can do the PS4 can do just as well with the added benefit of a meatier GPU.
The main difference between DDR and GDDR is bandwidth. For quite a lot of types of games that may not be as relevant as many other factors. My CPU point is that if a game has a bottleneck that is due to some CPU process then the other factors are irrelevant. AI and a variety of other elements are becoming increasingly important and will make up a ever increasing element of a games performance. GDDR isn't going to help that all that much. When you have a CPU or other bottleneck the meatier GPU stands idle and provides no extra benefit. Its not a case of automatically giving developers more to play with.
If developers want to make graphically intensive games they will do so on whatever platform their publisher tells them to. They might have to be more inventive in technique on one platform or another but no publisher will ditch making money on a given platform if there is even a sniff of cash there. When there have been exclusives for one platform or another the reasons are business driven rather than technology driven when you look at platforms of a similar generation.