Technically, 80/20 is usually considered a brass, with 80%copper and 20% zinc. Phosphor bronse contains up to about 15% tin (the remainder copper, but no phosphorus) which is slightly more corrosion resistant in many cases.

The physical properties of the different metals don't vary much as far as the sound of new strings are concerned. The real differences occur in the relative size of the steel core and the alloy winding, and in how closely the windings are spaced. With use the space between the windings gets filled with fingertip debris, and the alloy work hardens, both of which make the strings get stiffer and thus affect pitch and intonation.

I tend to change mine whenever the higher harmonics start sounding dull, but it's really a matter of personal taste.

~S~