Both have their respective uses. A fixed-wing carries far more water (3,200 gallons for the Be-200 tons vs 100-700 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. It's quicker to load up and discharge near the water, but a fixed-wing flies faster, so is better suited to delivering water 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.