Welcome aboard Shirl !
Your question about which valve to choose is one of our hottest topics. See the VALVE SELECTION Forum for lots of discussion on that topic.
The latest data on the Bovine Pericardial Valve is 90% durability after 15 years with several early recipients approaching 20 years. The GOOD NEWS for you at age 73 is that tissue valves tend to last longer in patients over age 60 so IMHO, this is probably a good choice for you, unless you come from a family with lots of 90 year olds. That valve was my FIRST choice, but due to other factors, my surgeon felt he needed to give me a mechanical valve.
For issues concerning surgery, see the Pre-Surgery and Post-Surgery Forums. There's LOTS of good reading to be found here. ENJOY and feel free to ask any questions that come to your mind.
'AL Capshaw'