To a large extent, hype influences the first week of sales while word of mouth influences everything after week 4. The period between week 2 and week 4 is a bit hard to define, as it's largely influenced by both.
The absolute vast majority of games sell more than half of their sales after week 4. There are obviously some exceptions, but not a lot. The Call of Duties are such an exception, with Black Ops actually selling near two thirds of their sales in the first month.
Whether word of mouth or hype is more important depends on the genre of the game. If it's a shooter, hype is more important, simply because so many new shooters release that compete directly with it. It's simply incredibly hard for a shooter to maintain very high sales for a long while due to the excess releasing of shooters.
For most genres outside of shooters, word of mouth is more important. Nearly all games that sell well (more than 1-2 million) have the majority of their sales coming after week 4.
It's also worth mentioning that nearly every single title that makes a franchise into a success relies nearly completely on word of mouth.
For shooters, it's hard to say which is more important for lifetime sales. For the majority it's hype, but some gain immense sales from word of mouth (Call of Duty: Modern Warfare comes to mind)
For non-shooters, word of mouth is practically always more important.
