Good readability helps achieving perception. Rich formatting is very important for issue tracking software as it allows you to distinguish and highlight more important things thus drawing special attention to them. In Redmine, rich formatting is done using Textile syntaxes, which is supported almost in every text area. In addition, Redmine provides formatter API, which means that the default Textile formatter can be replaced.

There were attempts to add different syntaxes to Redmine Wiki but the only successful and still active non-native Wiki syntax plugin is the Markdown Redcarpet plugin (https://github.com/alminium/redmine_redcarpet_formatter). This plugin replaced the default Redmine formatter with the Markdown formatter, which uses the Redcarpet library. This is the same library that is used by Github. Therefore the syntax provided with the Redcarpet plugin is compatible with Github's Markdown syntax.

If you asked what formatter I recommend, I would answer Textile. The Textile formatter is native, well supported, and well tested. However, when choosing a formatter you should consider your audience; if your audience is mostly Github users you should perhaps try Markdown but still remember that the majority of Redmine installations use Textile and users may expect your Redmine to use Textile as well.

But let's not be too verbose and compare basic rules of these syntaxes: