I'm not yet interested in going down the expensive lenses road in an endeavour to get closer to my pelican chicks but this may change with more time and experience. A post on birdphotographers.net seemed to explain the differences well.
"......... both teleconverters and
extension tubes allow you to project a bigger version of the image on
your sensor, but they do it in different ways. The teleconverter is a
lens which bends the light coming from the lens in front, thus
magnifiying the image, and you do all this without having to get any
closer to the subject. In contrast an extension tube is just that, a
tube, and not a lens, and all it does is give you a closer focusing
range. So an extension tube allows you to project a bigger image on your
sensor but you do this by getting closer to your subject. And of course
you can use a teleconverter and an extension tune stacked together.
Another difference is that teleconverters, because they are lenses, have
the potential to degrade your image whereas, an extension tube is
unlikely to do this. Yet another is that at least Nikon and Canon
teleconverters will only fit on some types of lenses, whereas extension
tubes fit on all lenses as far as I know. 1.4 teleconverters will cut
your light in half but a relatively short extension tube does not affect
light levels that much.
For macro work, a teleconverter allows the capture of a larger image of
your subject from a greater distance than an extension tube would, all
other things being equal. This might be useful for subjects like
butterflies or frogs which can be skittish. For macro work on flowers
that don't move, an extension tube might be a better option, where the
utmost image quality is critical............."