Let's talk about differences between 303, 304, and 316 stainless steel rod, and 6061 aluminim. All in 3/8 inch diameter.

Here is the use case. I have a reproduction 11x14 foot medieval pavilion that has 18 perimeter poles and one center pole. The perimeter poles have pins - these stainless steel pins - that are 3/8 inch diameter and each go through a grommet in the canvas, then a rope is looped over the pin and staked down and tensioned. This keeps the tent upright and generally in shape. The wind and rainstorms that come and gust at the tent are of course the major strain on the pins, but also there is a shear force across the pins as the rope pulls out and down while the canvas pulls in the opposite direction, through the grommet

It is not a marine environment, but rain does happen. I basically don't want to deal with the rust of the current mild steel pins, so a bit of oxidization is fine as long as it isn't super messy 

Which material would you recommend as best choice?