XML or .txt (flat file)

This is just a curiosity question for those who have experience with Flash and PHP. When doing a loadVariables() in Flash to a PHP file, is it quicker for PHP to write to a XML file or to a .txt file (aka flat file, I think)? What about vice versa? Would it be quicker for Flash to read a flat file or a XML file?

I would appreciate any and all opinions, suggestions, comments, etc. about the two.

Thanks