What is the use of a single ultrasonic signal?
I'm not an EV3 expert, but my understanding is that continuous mode stores results into a buffer that can be read nearly instantly. Single mode will need to wait to hear a ping back before reporting results.

Continuous mode will give you a faster response time. However, if you are using a number of ultrasonic sensors, they may interfere with one another, and using single mode should prevent this.