Chemical Composition:
304 Stainless Steel: 304 stainless steel has an austenitic structure and typically contains around 18% chromium (Cr) and 8% nickel (Ni). Therefore, 304 stainless steel is also known as "18-8" stainless steel.

Duplex Stainless Steel: Duplex stainless steels contain both austenitic (50% or more Cr and 30% or more Ni) and ferritic (50% or more Cr) phases. They typically contain 22-25% Cr, 4-7% Ni, and 3-5% Mo.

Mechanical Properties:
304 Stainless Steel: 304 stainless steel has good tensile strength and is commonly used. However, it may exhibit hardening tendencies at high temperatures.

Duplex Stainless Steel: Duplex steels have high strength and durability. They perform well at both high and low temperatures.

Corrosion Resistance:
304 Stainless Steel: 304 stainless steel exhibits good corrosion resistance in general atmospheric conditions and mild corrosion environments. However, it may be less resistant in more aggressive chemical or salty seawater environments.

Duplex Stainless Steel: Duplex stainless steels offer significantly higher corrosion resistance, especially in aggressive corrosion conditions like salty seawater, chloride-containing environments, and acids.

Price:
304 Stainless Steel: 304 stainless steel is more common and economically accessible.

Duplex Stainless Steel: Duplex steels are typically more expensive due to their higher alloy content and increased strength.

In general, 304 stainless steel is a more commonly used material suitable for general-purpose applications. Duplex stainless steels are designed for more demanding conditions and are preferred for industrial applications that require high strength and corrosion resistance. The choice should be based on specific application requirements and budget constraints.