warriors armor

i've been working on my frist attempt at makeing a warrior im choose to make a w/e im curently useing fire spells "conjure fire and inferno" i just made it to droknors with him and i was wondering about the armors theres knights arm which has reduced dmg and plate mail which add +5 def on each piece so really whats better arnt both effects basicly the same it doesnt say how much dmg it reduces just seeing if any one can give me any advice on which one is truely better....or exactly how much dmg is reduced by each piece of knights armor

(btw) im only useing fire because i found a pretty sweet firey dragon sword with my farming monk...i plan to use water spells once i find a cold dmg wepon