PNP vs NPN I/O and PLC

Please read this article and tell me what you think.
I am a little bit confused on The NPN vs PNP I/O and PLC. Maybe I am reading it wrong but reading this article and also reading the online tutorial on DC Inputs from Phil got me mixed up. I was wondering if I can get a clear explanation on the subject. Can somebody direct me in the right direction (Reading, tutorial, etc).
Thanks

http://www.patchn.com/npnpnp.htm

The emphasis in that article is safety. Using a negative/common for an input can create problems if the wire shorts to ground.

PNP vs NPN:

PNP can be remembered as Positive Negative Positve
A PNP PLC or I/O requires a Positive or Plus voltage as an Input. A PNP Output pushes out a Positive or Plus voltage.
PNP is the preferred "normal" way to go.

Best Regards