It's a 3-year gap in terms of their ages through their first AHL season, not a 2-year gap. You're measuring Rafferty by his age during his season and Rathbone by what his age will be at the end of his. They're both May birthdays. 3 years is a big deal.
Rathbone is far and away the better player.
e: and to respond to your edit, there are things you can look for outside of basic numbers and statistics to determine a player's likely worth before they even enter the NHL, such as their skills, technique and awareness/IQ; Justin Bailey does not pass the eye test, whereas Jack Rathbone has exceeded in it at every level.
And it's not really a 3-game sample size considering he played 61 games at the collegiate level before turning professional -- games in which he excelled all the same as he has been doing in Utica. Players exist before they enter the professional leagues, and there are ways to evaluate them independently of raw statistics.