The 2 doctypes you have listed appear to be the same? On the output tab for Sculptor, you can choose what doctype to use, it just defaults to XHTML strict. In general, I like to code XHTML strict myself since that should help with browser compatibility. If you are not used to strict, you can try Transitional and see if that cuts down on the errors.
A simple web search will give more info, but here are some basic differences between XHTML and HTML 4:
- all tags and attributes should be lowercase
- all tags need a closing tag (i.e. <img> would be <img /> in XHTML)
- the structure is a little more strict, the validators will catch stuff like that for you