Don't forget the costs of using a "lightweight" markup:
You have to learn some new system of quoting and escaping. I don't know textile or markdown, but I just went over to http://textism.com/tools/textile/index.php and tried to put a </code> close tag inside the sample code. It's got problems.
You get approximately one chance at a correct conversion to a richer format: converting to html. Nearly any other conversion, such as from the wrong lightweight markup to another lightweight markup, is lossy.
These markups aren't very extensible. They can't store metadata (e.g. microformats), or if they do, it tends to be by allowing html tags mixed into the markup, which means we're back into quoting/escaping issues.
There tend not to be any WYSIWYG editors, neither for you if you one day want one, nor for a less technical user who you'll someday want to edit your document.
So given almost zero knowledge about your problem, I'd recommend a WYSIWYG html editor. Whichever one you choose, it'll occasionally screw up and corrupt your formatting. Or maybe there will be subtle differences between the preview layout and a final browser layout. If you think those kind of issues will be a problem, then you REALLY don't want a lightweight markup language.