if u'r not going to the track or racing, then a metallic brake pad is a bit of an overkill...the semi metallic will do just fine. some metallic brake pads need to be brought up to operating temperature, so it's actually unsuitable on the street. even if u do go track/autox, the semi metallic should be good enough.

generally, the more metal in the pad, the higher the coefficient of temperature, or higher temperature capabilities. however, u'd also get more noise, more dust, and more rotor wear. semi-metallics are a good compromise for even most track purposes. metallics produce this nasty black brake dust that is actually corrosive on the rims. metallics also may not work as well in the wet. if u really wanted metallic, try metal matrix, these are streetable.