Difference Between 2-lead and 3-lead piezo discs

I am considering doing a project to detect the position of footsteps in a room using four piezo discs in the corners and seeing if I can determine the position by time difference in the signals. I don’t know a lot about piezo discs, though. The first thing I see is that there are two-lead and three-lead styles available on eBay (see images). What is the difference? Thank you.

PiezoDiscTypeA.jpg

PiezoDiscTypeB.jpg

I see this article says the 3-lead version allows for a very simple buzzer application:

http://makezine.com/2009/12/03/ask-make-three-legged-piezo/

Since I want to use this as a sensor, should I care? Should I just get the 2-lead version?

Just stick with the two wire type. They are what you want. The third wire will only reduce the signal you get from the vibrations.

Not sure if the whole setup will work, as I don't thing there will be enough signal to see from all four sensors, but try it.