I switched from a 1/2 to a 3/4 hp motor recently and I can tell you that there was a big difference.  The 1/2 hp motor had a tendency to slow down when you put any kind of heavy pressure on the ball(wet sanding or polishing seemed to do it).  It wasn't anything you couldn't work around, but it could be a hassle.

I found an equation for you to calculate the torque you would get with each one.
hp = (torque X rpm)/5,250
hp=horse power, rpm=revolutions per minute

The 3/4 hp motor hardly skips a step when you apply pressure.  That is the torque of the motor coming in to play.  The more power output(HP) an electric motor has, the higher the torque tends to be(given that the rpm stays the same).  As more resistance is applied to the motor, it will lose rotational speed slower.  If 1/3 and 1/2 are your choices, I would go with the 1/2 if it is possible.  It will net you about twice the amount of torque as the 1/3 hp motor will.
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If you want to be a legend, then bowl with one--Legends/Lanemaster

Davholio

I have a 1/2 hp Vertex spinner and though I dont use it alot, I have never been able to stop the ball from spinning. I agree with legendforlife95..


Shipper

Make that number 4 for the 1/2 hp Vertex.
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Equal opportunity annoyer

I went with the dual speed From DONKEY. the neck below the bucket was the selling point for me. This is the stress point where some spinners seem to break, from too much torque. handmeDn