The plastic tanks are much lighter, and could have a tendency to "float" in saturated ground if you have the tank pumped in the future. (as you should every 2 - 3 years)
A concrete tank will have less tendency to float, and generally will not rise up out of the ground if the tank is emptied and the ground around it is saturated. If that is not a potential issue with your situation, go for the plastic.
Good Luck!
Dodgeman