I was thinking 316 but I need to do more research and see the price and availability when I am ready for it.
316 stainless is the better grade. (304 stainless rusts quicker, but both will rust.) Anodized aluminium will last longer than stainless of either grade but will most likely be more expensive initially. If you use it in t slot you catch up much of the cost in reuses later whereas welded stainless is likely not able to be reused.
If using aluminium with any grade of steel or stainless screws it is best to have a barrier between them to prevent electrolysis of the aluminium.