In my limited experience with P-lips, I have found that many of the older system standard ones were narrower, and longer. For me, that made them "roll" a little bit and was a nuisance. I find the newer ones to be generally wider on the system standard, which make them feel "flatter" and sit better for me - no "rolling" and therefore more stable.
Also, I have found that the newer system standard stems are shorter and more similar to one another. Example: older 312's, 309's were longer; the newer 312 seems to be the same as the 314 stems.
I have also found that I really enjoy the same on the current system spigot series.
Short army was also great, I would like to get more of those.
Clear as mud?
The perfect size is the one you like best.