If you want to use a mark-up language in a chat app, a subset of either would serve you just fine and both are equally simple. It sounds like you are comparing a subset of what-is-markdown-today (and there are still variants) to full reST, which I don't think is a fair comparison.
From a very brief look, here's some things that probably aren't generally necessary (and aren't included in Markdown): Document structure, Bibliographic fields, complex footnotes and citations, directives, substitution definitions
All of these increase the number of things I have to escape, as well as my cognitive load when writing documents. And that's before we talk about reST config settings!
You could use a subset of reST, or you could just use Markdown!
If you want to use a mark-up language in a chat app, a subset of either would serve you just fine and both are equally simple. It sounds like you are comparing a subset of what-is-markdown-today (and there are still variants) to full reST, which I don't think is a fair comparison.