The 7.2V batteries are the ones that go on the robot - And they last forever (for me at least). I really hope that VexLabs starts selling the 9.6V Batteries individually soon. They always die many times over before I have to recharge the 7.2 on my robots.
I haven’t looked yet, but I also hope that the battery straps for the 7.2V batteries are now available separately from the Power Kit.