Both lens are nice lens to have. For 50mm, there is a F/1.4 version and the cost of that lens is similar to the 85mm f/1.8
I have the 50mm f/1.8. The cost of that lens is a lot less expensive than the 85mm f/1.8. The next lens I would like to get is the 85mm f/1.8.
I found that when I am outside taking pictures of my wife or daughter, sort of like portrait shots. I prefer a longer range. (longer focal length) But the current lens that I have can only do F/4 at that focal length.
Apart from the build, better autofocus ..... Since it is a longer focal length lens, it will have the less perspective issue.
For example, if a person lay down on the ground with the foot point away from the camera at an angle, the shorter focal length lens will have more issue of the person's head is a lot larger than the foot. And of course when the person is sitting on the grass and the foot is closer to the camera, the foot MAY appear bigger.
Of course, it really depends on the situation and how you frame the subject such as how much the subject take up the whole frame.
Don't get me wrong, both are nice lenses to have. It really depends on what you would like to do. If you want to do a lot of indoor head short type of photography, 50mm of course is going to be a lot more practical unless indoor mean inside a big warehouse. But then again, for less than $90 brand new, the 50mm F/1.8 is a great buy.