The easiest way to validate ping is to demonstrate that measurements made with it correlate with application response. Such a correlation is clearly seen when comparing HTTP GETs and ping responses. The two measurements are complementary. The goals of skitter and PingER are different. Skitter makes the measurements on roughly an hourly basis whereas PingER is on a half hourly basis.
PingER only makes measurements to the end node.
In many senses time is nonsensical for skitter measurements in most cases since it is not possible to obtain a path measurement instantaneously without simultaneous access to all IP devices along the path, skitter tends to provide snapshots. I do not see any long term archive of measurements that can be accessed for further analysis. That said skping does have some very nice canned visualization tools and has paid a lot of attention to the timing. PingER lets the monitoring sites choose their own measurement engine, which is often shared with other tasks and the monitors are usually managed by the monitor sites. You have to be a paying Caida member to have access to skitter and skping, whereas access to the PingER tools and data is free.