As with any licence, I suppose, it is whatever you label with that licence that it becomes. A single thing (e.g. website, software program, etc.) can include parts that consists of multiple licences, which means the whole 'thing' cannot become one licence, unless altering one of its 'sub' licences does not violate that licence.
On Wikipedia, for instance, the software, i.e. MediaWiki (both server side and the default skins) is GPL, but the content (e.g. text, custom CSS, images, etc.) is CC-SA as you correctly noted. Unless, of course, wherever stated (a lot of images have a variety of licences).
Essentially, no licence wins, because if they cannot be converted to one another, your website has to be released under several licences. However, in general terms, a website appears under one licence, unless noted otherwise. As such, you may wish to include with your Wikipedia excerpt that it is CC-SA content.
I have no idea how much sense this post made, but essentially, it is not uncommon for a multitude of content to have a multitude of licences, even if within the same 'scope'/website/etc.