Are aluminum anti-seize and silver anti-seize different? I asked someone to pick up some silver anti-seize (which I'd seen recommended) and they came back with aluminum anti-seize.  Is there a difference?  

I could imagine that, when used between aluminum and steel, you wouldn't want aluminum in the anti-seize.
Anti-seize compound exists to prevent galling, which according to Wikipedia is a problem with metal parts in sliding contact, particularly parts under high pressure with stationary or slow-speed contact.  Some metals, such as aluminum and stainless steel, are more prone to galling than other materials, like hardened steel.

Loctite make a brand of anti-seize compound, Loctite 37565, that is marketed as "Silver Anti-Seize", that they say is "fortified with graphite and metallic flake"; they don't say anything about the product actually containing silver, so I think "silver" is a marketing term.  The metallic flake in it might well be aluminum (copper also seems to be popular).  I couldn't tell you whether aluminum or "silver" anti-seize is better, but I do know from personal experience that if you're using stainless steel fasteners, any kind of anti-seize compound is much better than none.