Now that's a dam good question. A few powerful punches will put a guy down quicker but many fast punches will keep them off balance, overwhelmed, on the defensive, and it will be easier to nail them. I guess it's just a matter of personal preference and body type, attributes, etc.
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Member of DaJoGen MMA school under Dave Hagen and Team Chaos fight team under Denver Mangiyatan and Chris Toquero, ran out of Zanshin Martial Arts in Salem Oregon: http://www.zanshinarts.org/Home.aspx,