Hi Trilogy Fan,
The goal is to always send in your best work - and best means that you've worked on that particular piece a LOT. As you know, plays are always a work-in-progress so sending in an "untested" sample is probably not to your advantage. Before you send it, have your actor friends read it out loud so you can hear it. Because from what I've learned of this process, the writing sample is what will put your application on the top above everything else. That writing sample could be the difference between you being a finalist and receiving a rejection letter.
I submitted a full-length play to all the programs that required one (the play I submitted has been developed and won several awards and contests.) Many programs also accepted one-act plays. NYU, for instance, wanted a 80-page max portfolio, so I submitted the first act of my full-length and two 10-minute plays. This requirement allowed me to show the variety of my writing which was great.
I hope this perspective helps.