Actually both are fairly important for the functions of absorption of nutrients and retainment of water, and the elimination of waste products and excess nutrients and water. The digestive system mainly is responsible for absorption of nutrient and water via the Small Intestines and Large Intestine's respectfully, and following this excrete feces, which access food product that is not able to be processed.
The Urinary system while not exactly a more absorption property, is more capable of keeping the fluid and electrolyte balance in check by eliminating waste and excess products. Additionally in the rare circumstance, you could absorb water and electrolytes from the Uria in the kidney's before it travels down the Ureters, if there is rapid electrolyte imbalance and dehydration. The urinary system is more known for it's excretory process, which would be the Urine that we pass.
