What’s the difference between the two?
There are no differences.
You can use both, .pitch and .p, I see no reason why you would ever use .pitch over .p though.
[editline]25th April 2015[/editline]
By the way, both of those wikis are outdated and contain a lot of incorrect info, use this instead - http://wiki.garrysmod.com/page/Category:Angle
I know I can use both to access the properties/fields. And I have readed all three known wikis - none of them stated any difference.
So, both (p and pitch) have existed in previous versions of Garry’s Mod?
As I already said, there are no differences.