Which DTD to use? (HTML vs. XHTML)

OOh, good question! There are a lot of opinions out there about this - even the experts can't agree.

First of all, it's always best to use a strict doctype if possible. This means that the browsers won't go into "quirks mode" and you'll get a more reliable display. I personally prefer to have a Strict doctype with a few errors than a Transitional doctype with no errors. If it comes down to that - I don't usually find it that hard to get things to validate strict.*

So that baiscally brings you down to HTML 4.01 Streict or xHTML 1.0 Strict. The problem with xHTML (and particularly xHTML 1.1), is the fundamental difference between HTML and xHTML and how the browsers serve them. Some browsers (IE basically) can't serve xHTML as xHTML, they serve it as HTML. Someone else could explain this better than me. There is some information on the Mozilla Developer's FAQ.

Anyway, some people think that it's a problem to serve xHTML as HTML and will therefore use HTML 4.01 Strict rather than XHTML 1.0 Strict. There is a lot of discussion out there about this.

xHTML 1.1 is not an option because browsers can't handle it correctly. It is also a misconception that xHTML 1.1 is the next version following 1.0. It's not.

* I'm not one who believes that validation is necessarily the most important thing. Mistakes happen. Just in case someone goes to my site and tries to validate it More on that here and here.

Forgot to add: I personally use xHTML 1.0 strict. Firstly because I'm not convinced that the HTML/xHTML thing is really a problem (more because I haven't researched it that thoroughly, not because I understand all the different sides of the argument). xHTML is also stricter than HTML 4.01 strict so you can't get away with as much

That is from one of those people who thinks it's a problem (or pointless) to serve xHTML to browsers that don't support it. I just prefer the xHTML doctype because it's stricter than HTML and the validator catches more problems (I might have to run some tests to verify that though because I'm honestly not sure how different they are in terms of what is/is not allowed. Just need to find a page with lots of errors to test on ...). The Mozilla Developer's FAQ that I linked above has some stuff that explains this. You can also do a search and find lots of debates. These are the two best known articles on the topic:

Sending XHTML as text/html Considered Harmful
Sending XHTML as text/html Considered Harmful to feelings

The second article is a response to the first one. I only have limited knowledge of the issues but I've read both of these and haven't come to a conclusion myself (which is one of the reasons I haven't gone back to the HTML doctype).

Whichever one you choose, definitely do switch to strict. IMO the strict/transitional part is most important.

Edit: Yes, the validator catches a lot more presentational attributes and such under xHTML compared to HTML. Here's another article that explains it:

http://www.robertnyman.com/2005/11/02/html-or-xhtml/

Thanks for all the useful comments.