Whats the difference between the metal spacers or the polyurethane spacers on a leveling kit?

Also is there gonna be some settling? if so roughly how much? I measured from the bottom of the fender to the ground and got 38" front and 40" rear. Im either gonna go with a 2" or 2.5" kit!

Thanks

 

the difference is ones metal and ones poly. Its up to you wich you perfer, mine are poly and i havent had any problems or settling.