The answer, as always to questions of this nature is the (rather useless) 'It Depends'.
The argument could be made that, because it is a mobile site this means that navigation is going to be intrinsicly harder, and therefore a more shallow navigation structure is preferable because that makes it harder for the user to get lost in the site structure - they would only be one or two steps away from wherever they need to go. However, the counter to this is that if you have a flat navigation then that makes the top-level navigation very link-heavy as you need to link to everything from the front-page. A navigation structure that includes numerous pages isn't pratical to display on a mobile screen.
The alternative to a shallow navigation is to have a very deep one, deeper than you would get on a 'desktop' site. This way you can have very specific and brief content occupying single pages, so there is less noise when the user reaches the desired content page. Each page would be concise and particularly relevant to the purpose. However the obvious counter to this is that it's going to take much more time to navigate and browse to these pages if you're using the in-site navigation (and not just hitting the page directly from a search result / shared link).
Just to come at your question in a different way altogether; why assume that you need to make a choice between one or the other? A navigation structure assumes that the user is going to need to navigate around the site. In this day and age, is that really true?
In the same way that the music industry has changed from forcing people to buy a whole album just to listen to the tracks they want to, to a more flexible 'buy the individual tracks you want and nothing else' so too the web is changing. People don't just navigate to content from within the website, they navigate to content by clicking shared links that are suggested on their social-media pages, or from links mentioned in other websites altogether.
When the user is on the page in your site, why require them to have to revert to traditional navigation methods to browse around? That's probably not how they arrived at the site. Why not take a different approach -
- set up tags against your content so the user can browse to other related pages.
- Include some 'you may be interested in...' suggestions within your page so they can go off and find other information of note on your site that way.
- Heck, even suggest pages on other peoples sites instead! Don't pretend that your site is the only place people can get information. Give your site added-value by becoming not just a source of good information, but a source of other sources as well so you can start to become a trusted voice. That's far more valuable than just being 'an easy site to navigate around'.
The web is growing, don't just assume navigation is something that needs to be a choice between two options that have always been around.