What is the Advantage of sync.WaitGroup over Channels?

Independently of the correctness of your second example (as explained in the comments, you aren't doing what you think, but it's easily fixable), I tend to think that the first example is easier to grasp.

Now, I wouldn't even say that channels are more idiomatic. Channels being a signature feature of the Go language shouldn't mean that it is idiomatic to use them whenever possible. What is idiomatic in Go is to use the simplest and easiest to understand solution: here, the WaitGroup convey both the meaning (your main function is Waiting for workers to be done) and the mechanic (the workers notify when they are Done).

Unless you're in a very specific case, I don't recommend using the channel solution here.

It depends on the use case. If you are dispatching one-off jobs to be run in parallel without needing to know the results of each job, then you can use a WaitGroup. But if you need to collect the results from the goroutines then you should use a channel.

Since a channel works both ways, I almost always use a channel.

On another note, as pointed out in the comment your channel example isn't implemented correctly. You would need a separate channel to indicate there are no more jobs to do (one example is here). In your case, since you know the number of words in advance, you could just use one buffered channel and receive a fixed number of times to avoid declaring a close channel.