Snider is 5-11 190+lbs who has grown into his body and can just rake. Power Potential isn't astounding, but he has great disapline and unlike most big power guys he doesnn't strike out much. He's probably closer to the majors than Rowell. But also he's not as good of a fielder. And he could be moved to a DH spot if need be. Think like corner OF/1B. Think like Brian Giles.
Rowell is 6-5, 180+ lbs, and has not matured into his body yet. He seems like he will get moved (like Ripken) from SS to 3B. His power is just starting and if he can match his size and swing, there's no telling how could Billy could be. He'll probably take a little bit longer than Snider to reach the majors, but his ceiling is much higher. I would watch his progression into this year, in regards to his power. Plate Disapline isn't a problem and fielding is adequete.