The difference between "selection" and "assignment" really isn't as stark as it may seem. In our day, if you stood high in your class and were qualified (physically, academically, etc.) for your first choice, you got it. If you had 20/20 vision, you had a REALLY good shot at being a pilot no matter where you stood in your class (this is b/c corrective surgery was not available at that time). If you were in the bottom half of your class, things got a bit more dicey. You often got your second or third choice b/c there were no billets left in your first choice. This was an issue with folks wanting to be NFOs as those billets went very quickly (all the folks with bad eyes that wanted to fly).
As a result, in my day, quite a few folks went USMA to fly. At that time, USMC promised that, if you went Marines and wanted to fly (and were qualified), you were "guaranteed" an aviation slot. So, some folks with ZERO desire to be a Marine but a lot of desire to fly went USMC. It rarely turned out well.
Now, there seems to be a greater effort to match mids with a service where they want to go and are expected to do well. Thus, if you're at the bottom of your class maybe due to academics but knock it out of the park with leadership, PT and performance at Leatherneck, you have a much better shot at being a Marine today than if you were in that position 35 years ago.
However, I would guess that, if you stand high in your class, you still have a much better chance of getting your first choice than if you're toward the bottom. So, a different system, but outcomes are probably similar in the end.
