The 172X and 173P is not quite the same monitor. 173P has better contrast with its 700:1 ratio against the 172X 500:1. Also, the 173P is configured entirely with software (analog mode) whereas 172X settings is with buttons as usual. For those using DVI this will not make any difference since manual settings are not used here.
There is also a design difference, 173P is white and a little clumsier looking than the 172X. 172X is black
I was on the phone with Samsung yesterday and they said that the contrast ratio will not make any noticable difference, the only thing deciding which monitor to buy is your design preference and if you're using analog connection that the 173P has this software configuration thingy.
Specs I've read states that 172X has a dot pitch of .289 mm and 173P the usual .264 mm. When I asked about this Samsung said that this must be a misprint since they have the same resolution and size and therefore it wouldn't be possible with different dot pitches.
The 173P will also be about $40-60 more expensive than 172X.