Aluminium tube or steel tube for yacht legs ?

[rod]

I'm planning to build two legs to attach to my 6 tonne yacht hull (one at each side of beam) . This will keep the yacht upright when the tide recedes so it balances on its keel. The legs need to be light but strong so I'll use hollow section. There will not be much compression on the legs. The main force will be a bending moment (and thus buckling). Internet searches have not yielded any lookup tables. Assuming similar weight per meter (and so different wall thickness), I have two general questions:
(1) is square hollow section stronger than round hollow section?
(2) is steel or aluminium tube stronger?
Thanks.

[Cake of Doom]

You can download the Tata Steel Bluebook. That's got all standard sections and associated properties for your perusal.

Rather than comparing alu properties to steel, I'd go for the one that will give you the least amount of corrosion headaches. Prolonged exposure to seawater and oxygen is not ideal for any metal.