Definitely aluminum. I have a 7 x 14 aluminum dump trailer that is 16 yrs old. Still looks new. Aluminum chip body also. Same holds true for it. Remember that at least half the time you will be empty so you will be saving fuel ,tranny,tires. No rust is a huge plus. A steel bed would never last that long. So as far as a little more up front cost I say aluminum is more than worth it. Oh yeah, my brother pulls an aluminum 6 x 12 with 5ft sides dump trailer with his Tacoma crew cab.