Prior to select the module or theme, there is much to consider. One who deserves to consider is the amount of usage of the module or theme we want to use, more and more users the more visible the quality of the modules or themes we wish to use, but you have to remember, many users it has not been definitely better than those that have fewer users.
In Drupal, there is a module in its development objective is to examine the module or theme installed on a website, this module name is the Update Status module. But the Update Status module does not know the status of any module or theme that was not installed via a tarball produced by Drupal.org.
The module is already included in the core of Drupal 6.x, and for earlier versions of Drupal, you must install it manually. The results shown give an idea of how popular the different projects are and may help you choose modules and themes for your own site.
To assess the popularity of a module or theme by simply watching from the amount reported by this module is not 100% valid, because new data obtained can be reported to the Drupal.org if the website has turned on or installing the Update Status module, but this is enough to give an idea of what is used on others sites.
You can find out, how popular modules and themes you're using by visiting Usage statistics for all projects page at Drupal.org.