I'd greatly prefer HTML - I post my stories to several archives, the LC archive is the only one that doesn't use it, and every now and again I notice things like line breaks in the middle of paragraphs, at least once lines that didn't have breaks at all and scrolled off the screen to the right, and so forth. HTML doesn't have these problems, because inside a paragraph the line breaks are automatically inserted to fit the page, unless you put a line break command in the text.
Most of the archives that use it restrict the commands to the lowest common denominator, ones that can't do much harm - typically "center", "italic", "bold", "line break" and "paragraph break". A paragraph without any text formatting should be only seven characters longer than a plain text paragraph - (P) at the start - I'm using ( and ) instead of angle brackets - and (/P) at the end. This would be (I) Italic (/I), (B) bold (/B), or (CENTER)centered - sorry, cant't remember how to do this on the message board editor(/CENTER), and this would be a
(BR)line break.
The reason why HTML often appears to be much more bulky is that Word and Frontpage both produce hugely complicated HTML which tries to micromanage every letter of the document. If you write it in a text editor and keep it simple there is virtually no file size problem, and the results are considerably easier to read.
RTF and Word documents should be avoided - RTF gives different results (e.g. what the fonts look like) on different word processors, and Word can carry macro viruses.
Incidentally, one of the sites I post to, Twisting The Hellmouth, keeps the documents as HTML but has the option to read them as plain text - no idea how they do it, but that site has several thousand stories archived and the conversion is done on the fly, not by keeping two versions of the story, so it's obviously possible.
