Generally in softer materials, bigger pitch is better.

For this kind of application though, some form of square/acme thread would probably be a more suitable application, but that can get expensive.
I think I'd personally go for finer, rather than courser, as the forces aren't going to be that big in the grand scheme of things. If it does strip, either remake the top bars in steel, or put a helicoil in the aluminium.