biggest benefit of an academy slot is that your vision doesn't have to be perfect at the end to go to pilot training. The AF just spent a bunch of $$$$ educating you, and will set the medical standards for entry at a level to accomodate XXX number of pilot slots. Might be perfect now, but after 4 years of college, might not be....
ROTC, you are competing for only a few slots with every other college campus in the land. And if you are fortunate enough to get a pilot slot, your vision slipping to 20-25 may take you out.
Guard slots are different. Most of the guard pilot slots get used by people already in the unit....so the health doesn't have to be perfect. But, getting hired straight off the street is rare. Possible, but rare