Interview with Ralf Jüttner:
http://www.pitlane-vision.com/fils-d...sebring-r.html
They are not using the R18 at Sebring because the car is still in development and they only have a small stock of spare parts. Only after the race they will start producing them in larger quantities. The fact that the new rules were released very late, also did not help.
He believes that the open R15 will have an advantage in traffic. However he also pointed out that the new cars will have a better fuel consumption and hence will do more laps with one tank.