The 18" WSM can run long time using a clay flower pot base.
The trick to the flower pot base is you are using the minimal amount of fuel to hold the desired set point. Water is a more forgiving medium to use as a heat sink but that comes at a price, fuel. It's not a huge increase in fuel usage, but when you are running 12 hours or more on a load, it matters. The trade off is water will bring a temp spike down quicker than a clay pot base will. Reason is water tends to want to stay as a liquid. It absorbs quite a bit of energy when changing states from a gas (steam) back to a liquid (water) and wants to naturally moderate the pit at around the boiling point (212*). A clay pot base is great for a heat sink and bringing pit temps back up, but less so for a temp spike. Reason is the clay pot base does not change "states" and is just a clay pot base acting as a mass only.
With good air management, you can run a clay pot base in a WSM for a long time on a single load of charcoal. A power draft (also called a ATC or automatic temperature control), it takes less damper skill to achieve good air management.
But there are some things you can do with a electric that you cannot do with a WSM. Lower temperature smokes are better managed with a electric heat element on a PID than with charcoal. Not saying it can't be done with charcoal and good air management skills. But a PID controlling a electric element is hard to beat for some tasks (and you can still get smoke from a pellet tray/tube or other smoke generator)