Welcome to our forum.
Usually a mechanical valve is recommended for middle agers since it promises to last the rest of one's life and not require another surgery. The downside is that it requires taking a blood thinner like Coumadin the rest of one's life. Many people manage very well with this; others struggle to keep their INR level stable and with other complications, such as the risk of stroke.
For managing with a mechanical valve, check all the past posts on the subject on this site and you'll get a very good idea of the pros and cons of it.
For alternatives, such as a homograft or other tissue valve, there are also quite a few here who have chosen that route, such as myself. A normal homograft lasts about 15 years or so, on average. The trade off is the risk of another surgery versus the risk of taking blood thinners over that amount of time, plus some peculiarities of each kind of valve. If you choose to go with a homograft, you need a surgeon who is experienced and confident with this type of valve, of which there are many.
There are also more or less experimental developments of new valves which promise to avoid both a lifetime of blood thinners and further surgery. I chose such a valve, called the CryoValve-SG, a human valve (homograft) which is treated in such a way that it promises to last the rest of my life (I am 50 and just had AVR two months ago). So far, it is going extremely well. There are other similar attempts to solve the further surgery vs. lifetime of blood thinner problem with both human or animal tissue valves and mechanical valves. For mitral valve replacement, I heard from someone on this site that the Cleveland Clinic has a new type of mechanical valve that doesn't require Coumadin, but I don't know anything more about it.
Hope this helps. Please keep us posted and we will be glad to help you and your husband through this process as much as we can. If Mike indeed has a few years before surgery, which is not uncommon, then you should take full advantage on this site and elsewhere to become as knowledgeable as you can, especially from those who've been through it, about all aspects of valve surgery and especially valve choices.
Good luck and Godspeed.