I use XHTML Strict for one reason:
I love the concept of separating the content from the container and separating the style from the container. By keeping everything separate, you have much more flexibility with what you can do with each part.
As for the DOCTYPE, it's a way to tell the browser what to expect and how to output it.
It also serves as a guideline to finding errors in you code with validators.
If you are starting, you might as well use XHTML, you'll be happy you did it in the future when you want to make changes.