Strontium is chemically like calcium, so it collects in bones, where it causes bone cancer and leukemia. Cesium is chemically like sodium, which means it goes everywhere in your body but also washes out pretty easily. Cesium 137 is a nasty gamma emitter, so it can irradiate stuff at a distance. Strontium 90 is a beta emitter, so you have to eat it, breathe it, or park it on your skin for it to cause trouble.
You are correct. I should have looked for the chemical similarities to non-lethal substances.