When deciding whether to build a separate mobile site or to build a responsive site, a number of criteria need to be considered.

First, it is crucial to know and understand the difference between the two platforms. A separate mobile site means that the mobile site is detached from the desktop site. The mobile site and desktop site might share some content, but because they are different, they have the ability to relate to different aspects of the business. On the other hand, a responsive site changes its appearance based on the size and type of screen on which the site is viewed. The layout of the site adapts to the viewing environment by changing different aspects, such as image widths, column layout and menu structure. The site itself is the same, but the code changes to support the viewing screen.

How do you choose which of these options is a better solution for your project? Consider the following points, listed in priority order:

1. Customer/audience needs and actions — How will your audience interact with your site versus the mobile site? Are the use cases concerning how they interact with the site very different? If so, you need to have a separate mobile site. If the use cases are similar or the same, then you can consider a responsive site. There are two things to think about here: content needs and functionality. Are the content and functionality different for mobile devices and desktop computers?
2. Business goals — Similar to the audience goals, consider whether your business goals are different for mobile and desktop. If they are different, what is driving that difference, and how does your mobile solution need to be altered so that both your mobile and desktop solution address all aspects of your business?
3. Development and maintenance costs — How often is content managed and updated? Does having to manage content differently in two solutions make sense? Can content be shared, or does it need to be original for each application? Does the functionality need to be separate, as well?
4. Does the technology on which your site was built support responsive? For example, do DNN modules support the effort?
5. How many different mediums do you need to support? A separate mobile site means you have to manage two sites. If you add a tablet version and a large-screen version, then this becomes unmanageable.

Finally, it would be remiss if SEO were not mentioned. Google and other search engines want to accurately index your site and match content to pages. This means Google prefers if all content is the same for the same URL or is clearly marked. So, if done correctly, either method can accommodate Google; however, Google slightly prefers responsive, because it is easier for content to be matched to a URL (per Search Engine Watch).

There are many factors involved when deciding if you should build a separate site for mobile or use a responsive solution. You can simplify the process by concentrating on the first priority — focus on your audience. By focusing on your audience first, the other factors simply help you determine how to modify the solution to suit their needs.