NiMh or LiPo batteries for unattended charging/usage
For a hobbyist project, I'd go with a NiMH option. They are significantly easier to charge (you can charge them with a commodity linear regulator (LM317) and a few passive components), and safer. The weight difference will likely not matter much- your robot doesn't seem to need to change directions very quickly, and it's not something like a quadcopter where every ounce counts.