You guys should definitely continue to base a guys power off his half-season debut. Seriously. Great idea.
In terms of overall power CURRENTLY, Snider and Rowell are nearly identical. In terms of power potential, Rowell is a good amount ahead of Snider. At 6'5" and just 200 pounds, Rowell could afford to add close to 30 pounds of muscle to his frame. With a sweet swing that puts natural loft and tons of backspin on the ball, Rowell could be a SERIOUS long-ball threat. I've heard 50 homer potential, which is a bit absurd considering he's just 18, but it's still been thrown out there. Snider will obviously continue to develop, but he's not going to get any extra help from his body, which is already very mature for his age. Also, his swing has a slight uppercut, something which worries people. He may be able to get away with a swing like that now, but as he gets to higher levels, pitchers will exploit his uppercut and his average will fall significantly.
Rowell's ability to stay at 3B will depend on how much weight he puts on and how it affects his speed. He has average glovework and has good instincts at the hot corner. He comes through the ball very well, has a good arm, and moves well laterally. If he can add weight without slowing down much, he should be able to stay at 3B for at least the first third of his career. If not, a move to 1B is probable, where his value will take a hit, but if he lives up to his potential, it won't matter much.
My main point: Everyone values Snider far more than Rowell, but both posted similar numbers outside of the home run total, and Snider is a far more polished bat than Rowell. Rowell has better raw tools, nor is it particularly close, than Snider; more power, better hitter. It is not an easy decision at all, but if you want a guy with good upside and average risk, go with Snider; if you want a guy with tremendous upside and high risk, take Rowell.