Choosing rangefinder for obstacle-avoiding I am looking to buy a range finder module for an obstacle avoidance robot. 
Currently, two good options based on available libraries and compatibility with Arduino and rPI are HC-SR04 and parallax ping. 

In terms of form factor these seem the same. The only difference I can tell is 4 pins vs 3 pins. Also the price, with parallax being more than x4 the price. As I would rather not buy something cheaper which will need replacing, is anyone aware of further differences, that will make the parallax a better option? 

both need 5V
both have max range of 3-4m
Sensor angle HC- 15 degrees, parallax cant find..
Odds are pretty good they are real close. I've only used the sr04 with
  4 pins (too lazy to mod) and it was ok. cant say if the parallax is
  better or not. For 4 times the price you'd expect it to be, but so far
  in robotics, price seldom reflect performance in my experience. – 
  Spiked3


I've also used the SR (with 4 pins) and it worked great. &lt;2$ on eBay