[![One margin is enough!][1]][1]
<!-- source: https://dl.dropbox.com/u/118359/stackoverflow/sm-two-margins.png -->

One margin is enough!

The defining characteristic of mobile screens is that they are small; every pixel counts!  A design with two margins is wasteful (and also adds extra visual noise).

[1]: https://i.sstatic.net/4slh6.png