Typical end users dislike raw HTML. Most of them don't know what a <code> tag is. Markdown (though probably not Textile) is superior to HTML for end-users in that you don't even need to know how it works to get better-than-raw-text results.
My company's product does Markdown, and my philosophy is this: tens of thousands of people edit Wikipedia. Most of them are not particularly computer savvy. Wikipedia has perhaps the most baroque and ineffecient markup syntax possible, and people still tolerate it. So, there's the lower bar. By all means, exceed it, but don't obsess.
I suggest an editor like vim or Emacs with HTML modes that give you auto-indenting and tag completion key bindings. It’s really not that much more work to author straight HTML, and if it’s what you’re targeting anyway, you’ll be happier in the long run when you have to maintain it.
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.