If i'm having a heart valve replacement do i have the right to insist on a tissue valve or mechanical valve?

Absolutely: Your doctor hopefully will apprise you of the risks/benefits of both valve types. You as the patient, armed with that advise, can make the decision. Often times, your doctor will recommend one over the other, but again, you make the final decision.

Sort of: You can discuss the 2 types of valves with your surgeon and even tell them what your preference is. If the surgeon disagrees you can see a different surgeon for a different opinion. It is possible you would find someone who would put the other type in. You cannot however demand a surgeon do a particular procedure or use a particular device if they don't think it is indicated.