Mechanical replacement valves have lifelong durability, however a blood thinning drug must be taken for the rest of the patient's life to prevent blood clots from forming on the hinges of the valve. Besides being somewhat of a nuisance, taking blood thinners also create a small risk of bleeding complications.
Tissue or "bioprosthetic" valves are made of cow or pig tissue. Because they are made of natural materials, there isn't a need to take blood thinners. However they are less durable, lasting about 10-15 years.
Your doctor will help you make a decision on which kind of aortic valve replacement to pick based on your age, occupation, lifestyle, medical history, and preference.