Outside of his size, I am not sure where I see how you come to that conclusion. Also, NFL prospect doesn't make a player the better player at all moments in time. Throughout their college careers, Osborn has been the better wide receiver. More productive. More versatile. As for NFL athlete...I will wait until the process starts in the offseason. He doesn't look explosive, but I'll wait on that until we get some concrete numbers. They are different players and the difference between the two is negligible. If a GM took Cager "high"-ish, that GM wouldn't have a job for very much longer.
If Cager floats around on a practice squad or as a WR4 for some bum squad in the NFL, it doesn't mean he's magically a better player throughout their college careers.