I agree with LB. A short, upright shoulder equates a short, choppy stride. A long sloping shoulder offers a much smoother ride. To be fair, while short and steep is the most awful, short and angled isn't much better. I find while the stride is longer the shorter shoulder makes the horse choppy. My horse has a very steep shoulder, yet it is also very long, his stride can be choppy if you try to slow him down, he almost lurches, but if you let him stretch out, he gets just as much freedom as a sloping shoulder and is quite smooth.
I think shoulder has a lot to do with discipline, any sort of riding where one would need a nice long stride like dressage or reining, you would want a horse with a long sloping shoulder. I think a short upright shoulder would be fine if you wanted a short upright stride (I'm picturing crab-walking WP horses lol) but I find that usually with a short upright shoulder the depth of heart girth is usually not great and the front end can be quite weak.