There is no proof that any tissue valve currently on the market will last 25 years. There is some speculation that Medtronics' specially treated porciine valves will retard calcification, but that is not the only factor involved in durability.
My background is biomedical engineering, and both Medtronic and St. Jude Medical are headquartered right here where I live in Minneapolis. This means I have gotten a chance to talk at length with the biomed folks I know, and while they are very optimistic about the extended durability of Medtronics' stented and stentless porcine valves, there is nothing but speculation about how long these will last. There is strong sentiment that the Mosaic and Freestyle porcine valves may exceed 20 years durability, but there is no empirical evidence to back that up.
I did consider the Freestyle Metronic stentless valve, but instead opted for the Cryolife aortic SynerGraft homograft. Since this valve has been demonstrated to repopulate with the recipient's own cells, and become your own living tissue, it has theoretically the highest chance of extended durability among the tissue valves; however, I must underscore "theoretical" since none of these tissue valves have been around long enough to have the proof of time.
Different levels of surgical skill are required for different valves. As a simple rule of thumb, a mechanical valve is surgically the simplest to implant; the stented tissue valves are nearly as easy, but the stentless porcine and homografts are more challenging. Not all surgeons are capable or experienced in implanting stentless tissue valves, even though these arguably give the most natural hemodynamics.
Everything I have heard about the Mosaic valve has been positive. If you choose it, I just want to make sure that you do so understanding that durability of ALL the tissue valves is speculative with many variables involved.
Best,
--John