Both are used by professionals, and the choice comes down to your instrument, style and preference. Many beginners start with nickel because it's balanced and versatile.
Stainless can seem louder because it sounds brighter and more aggressive, but actual output depends on many factors. On a 5-string, stainless may give the low B more note definition, while nickel offers a smoother, more balanced response. Nickel-plated steel and stainless steel are both available across our standard 5- and 6-string lines.
If you're switching material but keeping the same gauge and construction, you usually won't need major setup changes. Many players feel stainless keeps its brightness longer, but lifespan depends on much more than material.