stainless all the way !!! aluminium is not like most belive a non oxidizable metal.
in winter condition it will came dull. I know you want to powdercoat it but at that point you should use steel and powdercoat it.
most of us when welding stainless we don t use any precotion but for a sculture in stainless steel , there is some precotion to be done if you don t whant rust stain. Don t let any steel come in contact with the stainless or it may rust at that place of contact. when you finish the welding , there is some acide that you should apply to the stainless to neutralise it , it will clean your job and help to make it rust proof. when making the shipping package , dont use steel strap directly on the stainless, if you or they move the sculture with a fork lift or any machinery that use steel fork , you will need a wood shim or plastic between the fork and the sculture.
for grinding, use brand new disc , not old disc that have done steel. I wish it will help you.