Whats the difference?

I'm interested in getting one of the combo deal but not sure which one?
1. What would be the advantage with getting the 12020/9l over the 9920/9l combo?
2. I'm going to be running 12 cells most of the time so would it make a big difference? I have 6 spider diffs and a strobe setup with an11lb truck. I have a thread in the presales section with a pic of truck.