The ~~ syntax appears to work similar to *emphasis* where whitespace on either side deemphasises the text. For reviewing markup it is probably best to at least support the MultiMarkdown syntax or something similar as well (as we have been discussing in the reviewing markup topic) to avoid the whitespace issue. But reviewing markup is a separate topic and is best not spilled over into this topic.
Using -- for <s> is going to clash if there is an en-dash extension (see Pandoc’s smart punctuation for reference). I think we should avoid using -- for strikethrough/strikeout for this reason.
I’m in favour of using ~~ to represent “strikethrough” as this syntax is already widely used and looks like strikethrough/strikeout text. The question is what element should this syntax map to: <s> or <del>?