Text Links vs Image Links (SEO)

A slight departure from the normal SharePoint articles I normally write, so from a completely HTML design point of view, here's an interesting question that I've asked myself several times and only answered through my own experience and through quite a few assumptions.
If you have an image displaying text, how does that compare in terms of SEO (and by SEO I'm not referring to SharePoint search ranking here) against using the actual text itself in a link. To give an example, you may display a link using HTML markup like this:

<a href="/mylink/"><img src="thisisalink.jpg" alt="this is a link"/></a>

or like this:

<a href="/mylink/">this is a link</a>

Whichever way you decide, which one ranks better? 

It seems there is no definitive answer, but there are quite a debates about the whole subject. I read an article recently that helps to clarify: http://www.kingrosales.com/2009/04/seo-debate-text-links-vs-image-based-links/ but even this makes some assumptions. Having said that, in my experience, I have always used text and I think on the logic provided in the article, I feel as though that's probably the right way to go.