Which SEO approach is more effective is a simple question but the answer is not so simple. Before we can find the answer we should take a look at the search engine optimization itself: why it is needed, what it is and how it is actually performed.
The ultimate goal is to have pages that will be present on top positions for selected keywords. Top positions get most of the clicks which means traffic to your websites. Therefore, the first step is to choose target keywords or search phrases that have weak competition and high search volume. The next step is to create pages around these keywords. Of course, the pages need to be "optimized". This means that target keywords should be properly emphasized to make it clear what the page is about. This "emphasis" is called on-page optimization. Another phase of optimization is link building. Promotion of the site by getting or creating backlinks is called off-page optimization.
The purpose of on-page optimization is to make a page optimized or focused around target keyword. It is not possible to exactly specify what and how this can be achieved but there are some simple rules that can help to achieve the goal. The first prerequisite for any SEO is quality unique content (text). The webpage should look attractive to visitors and they should immediately find out what the page is about. Such pages are also discovered and indexed by search engines and they should also find the context of the page. To help them to quickly determine the webpage content we usually put the main keyword into page title, header tags and at other relevant places. Creating a short and attractive meta page description also helps. On-page SEO is pretty simple, you create a "normal" page and then check whether the target keyword is adequately emphasized.
Off-page optimization or link building is the second half of SEO. Each link to your page is not just a link which users can follow to reach your page, it is also a measure of popularity or authority of the page. Therefore, search engines take into account the number and the quality of backlinks to specific page in order to determine its relevancy. This means that you should spend some time to find link opportunities and create as many backlnks as possible. There are few important link parameters that you should consider like anchor text, nofollow attribute, authority of the page where the link is placed, etc.
Now you can conclude that both on-page and off-page SEO is needed to reach the goal: high rankings in search results. Both optimizations are effective and only when done together they have the most powerful effect. However, it is up to you to determine what is easier for you and for which part your will spend more time and will put more effort.