There is much debate on full vs abs vs relative in the web design world. And there probably always will be.
In terms of practicality, a full or absolute URL in a non-CMS system, binds you to keeping everything where it is, limiting changes. Relative URLs allow you to move things without a lot of typing fixes.
In a CMS system, full increases your character count and is supposedly slower. The truth of the matter is, there is no difference between a full url, absolute url or relative url. They're all dynamically generated anyway. It all comes down to what you need for your setup.
For instance, I happen to reuse a lot of aliases, so I use a blend of relative and absolute. None is any less stable. However if a CMS implements either of these based on different standards that are nonconformant, then it forces tricksiness or demands the use of other settings or factors.
Here are the standards in HTML:
Relative URLs are in relation to the current path position. If your destination is the same folder, there is no prepending /. If it is in a parent folder, you must prepend with ,,/ for each parent up you must go. This is where many CMS including MODx (in some special cases) go wrong, they often only include the alias, not the necessary path navigation.
Absolute URLs are always from the root of the site. They always should begin with a prepending slash. /joe/ is the page joe in the root. joe/ is the page joe in the current path (whereever you are). Some CMSes forget to prepend the slash, forcing you to use the base element (which is an optional element). Often, they force you to use it incorrectly (more below)
Full URLs bind everything from the Host and automatically include the root. In a CMS, this doesn't affect anything except character output. (If you're fighting for the extra X characters because your markup is too think, then your URLs aren't the issue).
On Base
From W3C:
In HTML, links and references to external images, applets, form-processing programs, style sheets, etc. are always specified by a URI. Relative URIs are resolved according to a base URI, which may come from a variety of sources. The BASE element allows authors to specify a document's base URI explicitly.
If a URL given has a prepending slash, it is not relative, it is supposed to ignore Base. (this is where browsers argue and BASE bugs occur). Relative URLs therefore should include their path. Here's the crux. MODx doesn't know what BASE you are putting in your document. So it doesn't include proper pathing properly for Relative URLs. To compensate, it allows you to override the URIs manually. In other words, MODx only works properly with BASE when BASE is [ [++site_url] ].
If the BASE is, say, your actual URL (which is an acceptable and taught and in many cases preferred), then all of your links are broken from MODx. You can fix this quite easily with a plugin.
Other Notes:
Note that makeUrl() (in Snippets) and [ [~ ] ] are not the same. makeUrl() does not use link_tag_scheme. It produces relative or full links depending on your Context and the target's Context.
Articles also ignores link_tag_scheme, and manually overrides all URIs, and can produce links that are contrary to your Settings and BASE. This is what causes many of the bugs related to navigation with Articles itself.
Final Notes:
Whether you should use full, abs, or relative depends on your document structure and your use of BASE. I bypass this entirely by using absolute URLs automatically. I only use relative URLs if I manually type them in.
