We just finished installing what I think is probably our 9th St John concave, unless I've missed one somewhere.    They will outlast a new one at least by a factor of two, maybe more depending on what you are  harvesting and how picky you are about trueness and a sharp edge....very important in wheat, not so much in corn, so the crop makes a difference in how long you can run one.  I ran one in my old 7720 for nearly 4000 hours, but it was not cutting wheat and I did weld up the front couple bars whenever we had the feederhouse off for cylinder bars. 

We used a set of rebuilt cylinder bars once.  On a Deere machine, I would not recommend it.  By the time you have worn the second set of teeth off of the bar, there is nothing left of it due to wear and they are bending all to heck.   

I won't put anything in but hardfaced concaves.  I don't like taking them out and installing them very much, I sort of like to get more then 1400 hours out of a concave once it's installed.   

If you are going through St John anyway it isn't going to hurt anything to stop and take a look.  They will probably have the concave for a 95 or 96 in stock, so you should be able to see what you are getting.   If you want to stop here on the way by I have a worn out one which needs to go back out there.   

I strongly recommend the hard faced concave.   I've run them for nearly 20 years and we have never had any problem with them, and they wear much longer then new.   If that's worth anything, take it into consideration.    Keep in mind that there's someone somewhere who has a beef of some sort with almost any business.