Which has a bigger impact? I don't know. Maybe that's the wrong question.
Maybe the question should be which one is healthy for the brand and the company and the customers. Long tail sales based on word of mouth are better for all involved. The long and short of it is: You can't sell based on word of mouth if the product is poor. Pre-release hype can help sell a poor product. That's not to say all hyped games are poor; of course hype can happen naturally with sequels. But if you have a crappy game and you need it to sell well, you better do a lot of pre-release advertisement. If you're relying on a few people to try and enjoy the game and then the word to slowly spread over months and months to make most of your sales, it better be a really good game.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
