Mainly, clip volume fades , or "clip gain" impact gain level. The initial level before reaching the channel faders.
Automation, as in volume automation will be the equivalent of moving your faders. Both impact volume, only gain being a little more critical for keeping a desired level passing through your channel.
They both have their uses, so explore what suits your needs. Speed in adjusting volumes, as well as adjusting sensitivity play a roll, so its one of those things best left exploring your own needs which will work best for you.
For example, gain adjustment could greatly impact a guitar level. So if the signal is too hot in one area, like a spike that is too high in level, it would be best served to reduce the clip gain at that spot to reduce that spike. If the signals were fairly consistant with plenty of headroom, and you needed to gain ride by moving a fader, then automation will be preferred. [Note: "gain ride" is often a poorly used term, as it's better suited to be called "level ride".]
There are no hard rules, but there are areas where you have to treat gain with a closer watch for headroom.
Automation will be preferred if you have the need to re adjust levels during a performance. Which can be more often. So its probably worth thinking of clip gain alterations as more surgically focused. And automation as more mix focused.