You right, metal burst is much better, I've never leveled an aggron past 50 so I never really paid attention to metal burst.
The reason I went with relaxed as its nature is because its speed is already so low and would much rather invest EV's in HP,S.def or attack(of cource defence is going to be maxed). Even with a beneficial nature and maxing out speed, its still going to be outsped most of the time,and even when it does go first it will only have its base attack, which is ok but not great. If it helps for aggron to go first I could also add him to a trick room team.
There other reason I went with relaxed was to take advantage of moves like avalanche or payback, but I think metal burst by itself should be good enough.
I think I'm going with this
EV's-252 def/4attk/252 hp or 252def/100attk/156hp