D3 is really boring after the first day or how ever long it takes you to get through a play through. You can get more life out of the game if you play it on hardcore, but it is really just a grind. D3 feels the same no matter who you play pew pew run pew pew.
In DS3 the combat is tactical on harder difficulty which keeps battles fresh and tactics change depending on what toons you are a playing. I would go with DS3.