There’s an interesting discussion going on about the merits of one over the other, here’s Dave Shea’s view, and here’s Cameron Moll’s take on it…
I’ve been using xHTML for a long time, but these two articles make a very strong case for reverting back to HTML4.01. I’m not quite convinced yet, I like closing my tags dammit! There’s a neatness to it, that’s the obsessive compulsive side of me! But in reality, using WordPress and Drupal as much as I do I don’t see any benefit in moving back at this point, who has the time anyway. I’ll keep my clean code and will go straight to HTML5, when….that comes out.