Should I choose the PocketLOGGER or KnockBOX?
This is only a question for 1G DSM and 3/s owners as those are the only cars that currently work with the KnockBOX.
In general, you can think if the KnockBOX as a gauge and the PocketLOGGER as a datalogger. The KnockBOX is not capable of storing data; it just displays data, whereas the PocketLOGGER both stores and displays data. The KnockBOX allows you to monitor your engine's vitals without having to have your PDA on. It's more of a permanent fixture.
One of the upcoming benefits of the KnockBOX is the ability to log parameters other than the parameters available within the ECU. This makes having both the KnockBOX and a PocketLOGGER very useful. The KnockBOX will connect to external sensors and allow the PocketLOGGER to read the values of the sensors from it, just like the PocketLOGGER reads the sensor values from the ECU. Currently these upgrades are in the works and no ETA is known at this time. See the KnockBOX page for more info.