Ping))) or SRF05?

They're really pretty similar. The SRF is a little better, but not by a huge amount. Here's a good informational page about the SRF05, and here's the product page for the PING))) which has a link to a useful PDF document.

The biggest difference is that the SRF05 has a range of 4 meters, whereas the PING's range is only 3 meters. Aside from that, the only other real difference is that the PING only has 3 pins so you have to use the same pin for input and output, whereas the SRF has 4 pins so it can operate in either mode -- either using the same pin for input and output like the PING, or using separate pins for input and output. The advantage of using separate pins is that it's a little faster -- when you're operating the SRF using a single data pin, it waits an extra 700 usec (0.7 ms) before sending its return pulse, to give your microcontroller time to switch the pin from output mode to input mode. So if you're using separate pins, you can get your results 0.7 ms faster. The PING has a similar delay -- it waits 750 usec before returning its echo pulse. And on the PING you obviously can't avoid the delay because it doesn't have the option of using separate pins.

Really, though, those 0.7 ms are pretty insignificant in the grand scheme of things. You shouldn't activate the sensor more often than once every 50 ms anyway, to avoid reading echoes from previous pulses, so the extra delay only makes it 1.4% slower at most.

So really, the range is the only difference I'd really worry about. In the US, the PING is available at your local Radio Shack, whereas the SRF has to be mail-ordered, so I'd probably go with the PING unless it's important in my project to be able to detect things between 3-4 meters away. But on the other hand, if you're not in a hurry (or if you're in a foreign country and have to mail-order it either way), you may as well go with the SRF, because it costs about the same and IS slightly more capable, and there's nothing the PING can do that the SRF can't.

Dan