I hate NPN devices
I have some older machines with NPN input devices, and I hate them.
We just had a short to ground on a cable and it disabled the door interlocks on a machine so when a guard was opened it did not inhibit the machine from starting. If this was PNP it would of blown the control fuse/breaker.
Back to the OP I was taught that NPN has a faster on/off time, and if you look at response times on prox or eyes you will see a slight differance between PNP and NPN, same with input cards.