I have been evaluating both for several years.
Subclipse was the pioneer of the svn eclipse plugins and has been around for years. It is a tigris.org project which is the same as svn itself.
Subversive showed up a few releases ago and appeared to be part of the eclipse installation process (available under Install new software). However, at that time it was very painful to set it up because you had to add another repo (polarion.org) for the connector in order to make it functional. Now, however, this is no longer required.
Subclipse was great for its time, but I like subversive a lot better. Specifically, I like the extra features it gives you in the commit window. It allows you to revert, delete, compare, etc each file in this window. Subclipse does not allow you to do anything but select/deselect each file you want committed and enter a comment.