what is the difference between a hydraulic cam and a roller hydraulic cam? what are the plus and minuses of the two?

 

a hydraulic cam has a flat cylinder lifter that slides against the cam lobe. see pic

a hydraulic roller cam has a roller bearing that rides on the cam this has less friction. it also prevents cam's from going flat as fast if at all and allows for a faster ram speed and longer time at max lift whitch give more hp. see pic