On Page SEO
On Page SEO is changes that you can do to your website to help it rank higher for certain keywords. This would include:
1. Optimising your page titles
2. Optimising your headings
3. Using search engine friendly urls
4. Using a search engine friendly menu system
5. Internal linking
6. Renewing your domain name for a few years ahead
7. Adding meta descriptions
8. Adding meta keywords
9. Image alternitave text (alt)
10. Keyword Research
11. Quality Content
Off Page SEO
Off Page SEO is basically getting other pages to link to your site. This can be done in a number of ways. Here is a list of the Top 10 Methods of Building Links to your site.
Now back to the question: Which is more important? Off page, or On Page SEO?
In a way, they are both just as important as each other. Off page seo is almost completely pointless without good on page seo, and on page seo is also almost completely pointless without good off page seo.
However, on page seo can be done to an extent where you have to stop where as off page seo is never ending.
Website Grader is a great tool to help you check your on page seo against your competitors.
Although I said “off page seo will never stop”, you may well get to the point with your website / blog where you have so many readers that are social bookmarking you, blogginb about you, linking to you, etc. that your time is better spent writing good content then doing any kind of off page SEO because when you publish new content, that is all done for you by your readers for free.
Overall, I think probabily on page SEO is the most important once your website is quite well established because, if you publish quality content, others will link to you with out you even having to ask them to.