Nickel is the most popular choice and is close to the brightness of stainless steel, but just a hair warmer. Many people prefer nickel because they are more adjustable than steel, which by its nature is much harder to adjust.
Brass is the warmest of the 3 metal choices.
The brightest of the three metals offered (stainless steel, nickel, brass). Due to the way they are manufactured and buffed to a high sheen, there is also less drag on the string, thus producing less pick noise and accents the brightness further.