You have to figure the cost, money and time. The spill bucket is easier and cheaper to build, but it takes a little more work after each spill. You will have to manually fill up a new bucket each time it gets dumped on its victim. So, in effect, the cost to build may lower and easier, while it is employed you are looking at a lot more manual labor to keep the event going. Although, some designs can automatically fill the bucket, but that might take a little more professional work:
On the other hand, a dunk tank works a little better. It may feel more natural and enjoyable for the victim to fall into the water rather than having the water dumped on their head. Also, there is no avoiding the water when they have to fall in it. With the spill bucket, they could easily step out of the way.
Perhaps you could put both together for double the fun. The victim could simultaneously have water dumped on their head and fall into water as well. That would take a very complicated design, but it very possibly to make with the right skills. I would venture to guess that nothing like that has ever been built.
I did see an amazing design for a swingset sprinkler system, which is not designed to get the swinger wet though. It shuts off right as soon as they pass by the stream of water. it looks amazing. There are probably Youtube videos out there that demonstrate it.
So, like this High School shop which has the manpower and resources you can build your own functional dunk tank. The dunk tank is very popular for summer time events. Many people enjoy the spectacle. It has all the aspect of safety mixed with humility and revenge. Get your boss to sit in the dunk tank or a competitor. Get someone in there who is a good taunter.
A summer event that involves the bucket spill and dunk tank can bring money for your charity. All you have to put in your advertisement to attract people is that a dunk tank will be present. People love to play with them, especially after having a few beers. No one should drown with as much attention as the event should get.
Why use water? if you are near a mud pit, you could have the victim fall in mud and then they come up dirty. Perhaps you could use 2 targets, one for a bucket of water and one for a dunk tank with mud. Do you want to clean them off with the bucket and make them fall in the mud? Be creative with it, there are many directions that you can take an event like this: