Real time interrupt you will receive from "real time" sources such as Real Time Clock (example DS1337).
External intrrupt can be activated by any event such as keypad, keyboard, sensors. They will interrupt the micro at any random period of time, whereas the RTC intrrupt will occur periodically at the same time intervals ..
External interupts are driven by so called events => event driven
Is some critical applications it is necassary to guarantee that e.g. every 25ms a adc value is measured and put out after postprocessing, like for controller => time or real-time driven