If you move your sites around, then use relative. If your page size is big, then consider relative. Otherwise, use abs, because, if they go to abc.com or www.abc.com they will see the exact links. Then you need to stick with www or without www from the begining.
I vote for the relative linking... it helps in keeping the page size down and if you move over to another domain (for some reason) you don't have to make changes to the links.
There is a difference between relative and absolute links. When crawler crawls a web page and a web page have absolute links on a web page then all absolute links will get indexed. Absolute links are good for SEO because this way a website's inner pages also get indexed when only one page of a a website is crawled. I would strongly recommend absolute links.
Effective backlinking that can boost your blog in the SERPS is a must if you want to do search marketing. But turning to on-page SEO, there is the important matter of internal link structure which is helpful for search optimization. If you are unsure about what is meant by this concept, then that is all right because we will make it all clear in a moment.