I believe there might be more variants than just two. In the future it might be more useful to talk about "useful isotopes," but for now check out this: https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html

You should check out what France does with their breeder reactors where they actually do fission on a lot of the fission products. So running your fission reaction actually produces more fuel that you can react again.