Links in general:
One problem with links is that they can die/expire, the pages on the target site get reorganised or deleted, and the link is no longer valid.
One other thing to consider is copyright, if you copy & paste content from another site, you may violate the rights of the original owners. From that point of view, a link is better, because it clearly indicates the original ownership (provided that the site you link to is the original owner, and did not copy & paste..)
Links on SO:
As TheTXI points out, posting only a link as an answer requires the person reading the answer to perform an additional action (click on the link) in order to see your answer.
That is why I try to do both: Put a link to my source and copy the most important parts from the target site or at least a small summary into my answer.
I also follow that approach for links to other peoples answers on SO (see example above), because I consider it polite to attribute ownership where it's due.