aluminum's fine. just take care of it. Stainless steel is simply more sturdy at the same wall thickness, and thus more durable even when not well cared for.
5gallons is ok for now but it won't be enough when you go all grain. (trust me, you will want to go all grain someday...we all do)
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10