Ranging Sensor vs Distance Sensor

What is the difference between these two items. How do they function differently? There is a big price difference as well.

Ranging Sensor $5 http://www.canadarobotix.com/sound-microphone/hc-sr04-ultrasonic-ranging-sensor

Distance Sensor $40 http://www.canadarobotix.com/sound-microphone/1611-parallax-ping-ultrasonic-distance-sensor

The most significant difference is the price. The operating principles are the same.

So why would someone purchase the item that is 8x more expensive in that case?

In my experience, the Ping sensor is much more reliable than the SR04 sensor.

I'm not sure the difference is enough for me to purchase Ping sensors at full price but I always look for them when they're on sale.

I've purchased several off ebay for $5 each. Apparently RadioShack sold Ping sensors at very low prices. The ones I purchased off ebay had RadioShack packaging. I found a few Ping sensors at our local RadioShack for $7.