Q:

Which are more efficient in putting out wildfires: planes or helicopters?

A lake is near where both helicopter and plane can access the water and fire.
Which aircraft (if you had access to only one) is more efficient in putting out wildfires? Which aircraft is preferred to fly for this? What cost less to operate and which aircraft can deliver the most water per load and on the average?
I have added this link below on how this airplane delivers water. Sorry it is longer than 4 minutes. In the video at 4:49 the plane can touch down and refill without stopping.

A:

Both have their respective uses. A fixed-wing carries far more water (3,200 gallons or 12 tons for the Be-200 vs 100-700 gallons for helicopters), but takes more local infrastructure to operate, and is more difficult to discharge exactly where you want it. A helicopter is more precise, but carries less water. A heli is quicker to load up and discharge near the water, but a fixed-wing flies faster, so is better suited to delivering water or a custom mix a longer distance away. 
You want both, not just one. You'd use helicopters to suppress smaller fires if possible before they get out of control, attack larger ones at specific spots, and fixed-wings to lay down longer lines of fire retardant against developed wildfires. Larger helicopters also support fixed-wings in this. Which is "better" if you could only have one kind... that's not so much an aviation question as a fire-fighting one. 
As for cost, helicopters are normally less cost-effective, being far more expensive to operate per ton-mile. A large aircraft is more expensive in absolute terms. Being smaller and less demanding of the infrastructure, helicopters can be stationed in a more critical location and react more quickly.
It's not a worse/better question, it's tools for the job. Fixed-wing fire-fighting aircraft are more specialized, with their internal water storage, while fire helicopters can be as basic as a light utility heli carrying a water bucket.

A:

It depends on what you are trying to do.
The planes aren't doing direct attacks.  In other words, they don't usually drop on fires.  They lay down lines of fire retardant or water as a fire break.  The fire burns up to the line, slows or stops, and ground crews can go in and stamp out hot spots.  The big Erickson helis can also carry retardant or water and perform a similar function.  Since helis can be more precise, they can hit specific hot spots and cool off the fire.
Direct attacks can be used in some cases, but it's not really a great way to extinguish a fire.  Actually putting a fire out isn't going to happen with a plane or a heli.  They may succeed in cooling the blaze but they probably won't stop it.
Simple version: the fire goes out when it runs out of fuel.  It runs out of fuel when there is nothing left to burn.  A bomber doesn't carry enough slurry or water to snuff out a fire that is worthy of national news coverage.  The strategy is to contain the blaze with retardant or water and allow it to burn up to lines and let crews and fuel exhaustion take care of it.

