I have an aluminum
arch on the back of my boat. It is a very common material for fishing
boats and catamarans. Possibly related to the comment above about Florida
is cheaper and lighter for the same strength.
Aluminum is weaker for the same size, so aluminum structures are built out of bigger stock to get equal strength. This is really an aesthetic issue. A small boat might look funny
if the strength requirements demand 2" AL pipe. Of course you're not likely to hang a 14' RIB
from the stern of a 35' mono.
SST is shinier than AL. Aesthetics again, but also a price
factor since for many SST is more desirable.
Durability of the finish I think is equal. SST wants to be polished, anodized AL wants to be waxed. AL has to be isolated from SST fasteners to prevent corrosion
. AL can be painted or powerdercoated, but neither will last. The comments about AL being messy must be referring to raw AL.
For a Niagara 35 I think you want SST, but totally my opinion based on aesthetics. $10K is absurd.