It seems that both of these commands play a single audio clip at a volume specified in the arguments.
What scenarios should each be used in? The best I can guess at is PlayOneShot for HUD/Interface type sounds (close to the player) and PlayClipAtPoint for environmental sounds (bullet ricochet, door opening etc.). Have I got that right?
Well, in my 2D scrolling shooter, I use PlayOneShot exclusively, because all sounds are equally interesting to the player.
In a 3D game, most objects would have their own sound emitters just to keep to the composition model as much as possible. PlayOneShot is for things that you want to always sound the same - so, yes, often that would be interface effects. You can also use it for PA announcements, player-hurt sounds, congrats-you-opened-a-treasure sounds, whatever.