Aluminum works just fine, but is not as resilient over long tours, or rough road touring, IME. I have broken a Blackburn rear rack, but that was after a lot of rough roads, a heavier load and over the period of several months. The rack had been "repaired" with clamps on a few welds and a steel rod connecting one of the stays back together (again clamped in place). The rack was replaced with a used Tubus Cargo and the following year of touring over rough conditions was handled easily by the Tubus.
My touring bike nowadays has a Tubus Cargo for the rear and a Tubus Tara for the front. I find the racks superb. There is no sway (the aluminum Blackburn rack was not bad, but the Tubus is leagues better in this respect). The Tubus work brilliantly with Ortlieb panniers, like they are system designed to work together. I like the three strut design of the Tubus Cargo, versus the two strut design of Blackburn for the extra pannier support.
For a week long tour, aluminum will likely work perfectly well, but if you are buying new, I would recommend spending the extra and getting Tubus for value and functionality. If you already have an aluminum rack that has worked well for you sitting in your parts bin, that rack should do just fine.
IanA Alberta Canada