Cesium is usually a liquid at room temperature, while rubidium is always a solid at room temp. They both react violently with water, but rubidium makes cesium look like a sparkler in comparison. read more
This page compares OCXO vs Rubidium oscillator types and mentions difference between OCXO and Rubidium oscillator.OCXO, TCXO and MCXO are quartz oscillators where as rubidium, RbXO and Cesium are Atomic Oscillators. read more