Any question answered...

Stainless steel all the way for bolts. Titanium if you like, but watch the torque. Alu is too soft for stuff like that.
Pretty sure I read somewhere (CTC mag?) that stainless steel isn't great for bolts - the high levels of chromium make them prone to fracturing. High-tensile steel bolts may develop some surface rust and look a bit shabby, but it's stronger in the long run.