I've heard that once a rivet rust on an aluminum trailer forget it.
Rust will spread like wildfire. On the other hand, galvanized only
stays rust resistant for a small period of time even after washing it
down with fresh water after each use. I personally own a galvanized
trailer and my fishing partner owns a aluminum trailer. I've seen both
in action and have towed both. My advice: galvanized are good because
they don't flex which makes for a more sturdy feel when trailering.
However, take into consideration galvanized will rust faster than
aluminum I'd take aluminum. Make sure you buy stainless stell hardware
for your aluminum trailer. If you're not willing to spend money on the
stainless hardware buy galvanized steel. I will probably last about
the same amount of time in that situation and cost less.