Hello Mike. Welcome. Glad you are here. We can do much to relieve any stress or concern, help you ask great questions and guide you through recovery.
At age 52, last year, I had a bovine tissue valve implanted to replace a very stenotic (diseased) bicuspid aortic valve. While I am on the younger side to receive a tissue valve (hopefully going to last 15-20 years before needing another replacement -- mechanical valves, like the On-X, generally last a lifetime. Mechanicals (st. Jude's, etc. require anticoagulants -- popping a pill every day, the On-X possibly just taking an aspirin each day), it was what I first felt compelled to go with...mostly a gut instinct.
While choice is a very personal decision (tissue vs. mechanical or brand vs. brand) it is sometimes the case that the surgeon will cut you open, have a look, and decide there is a different valve that will fit better. If you trust your surgeon and he understands your personality and your lifestyle then it should be very appropriate to go with the valve he has the most experience with and the valve which best suits your anatomy.
To garner more responses to new questions in your short time pre-surgery, please be sure to start a new thread with each new type of question (ie, if you have questions about the actual surgery, or recovery, etc....best to start a new thread so more people will see it)
Breathe! You're in good hands here!
Marguerite