It's not a question of one or the other, just like it's not a question of quality vs. quantity, contrary to what many books and magazines will tell you. It's about balancing all the variables in order to reach peak fitness.
No matter what the distance you are running, both are important, as are long runs, weekly mileage, strides and other workouts to build your raw speed, and all other factors of training. If you're running shorter races, of course, your focus is going to be less on the longer stuff and more on the faster stuff. If you're running longer races, of course, your focus is going to be less on the faster stuff and more on the longer stuff. However, you ignore or skimp on any factor at your own peril.