Is there any real difference between using mouse.Origin and mouse.Hit.p?

There is a difference.

First let me point out that mouse.hit is a property of mouse. This is a CFrame. The .p property of a CFrame is the position of that CFrame. In other words you are getting the position of the hit CFrame by mouse.hit.p. Origin and hit BOTH return a CFrame!

The difference is that the origin is "where the mouse came from" (its origin) and the hit is the position where the mouse - uh, well - hits.

So what is this origin then? Where does the mouse start..?

Here is the answer;

local mouse = game.Players.LocalPlayer:GetMouse()
local h = mouse.hit.p
local o = mouse.Origin.p
local pos = game.Workspace.CurrentCamera.CoordinateFrame.p
print(o, pos) --> Notice that they are (almost?) equal! 
print(h)

Thus the origin is the same as Camera.CoordinateFrame :) (run above in a LocalScript)