[Tristan Lall]

Remember that comparing theoretical static stresses against material strengths won't be sufficient to guard against complicated load cases—like the ones you're likely to see in an arm that's in motion (i.e. has momentum) and may well be pushing or pulling on things (or being pushed or pulled itself).

The thicker material will be more resistant to deformation, because of the increased stiffness (the modulus of elasticity is roughly constant for all aluminum alloys), so you'll be less likely to suffer a dent that causes the member to buckle.

Also, depending on the details of the planned design, you may find it more convenient to lighten the arm by cutting strategically-placed holes, rather than making it from lighter stock.

We'll need lots more information to give you concrete suggestions about your particular design.