I am fairly certain that there is only enough for one sample (i.e., it cannot be split). I totally agree that I would rather have my own blood/cells than a stranger's, but in a life-or-death situation, I'll risk getting a stranger's!
If you were to donate all of it to public use, the other important consideration in my mind is: How long would it take to get a donation from that source if it were needed? It would be disastrous if God forbid the child were to fall ill and had to wait too long to get the donation. I would find out if there a waiting list for cord blood from the entity you are thinking about donating it to.