From a corrosion-resistance perspective, if 304 stainless steel is working for your urea piping now, 316 stainless steel should also work. The main difference between 304 and 316 stainless steel is that 316 contains 2-3% molybdenum. The molybdenum makes 316 stainless steel generally more corrosion resistant than 304. Schweitzer's Corrosion Resistance Tables (5th Edition) rates both 304 and 316 stainless steel as "good" (< 20 mpy) for 50% urea service up to 93 °C.