Well, I am the author of Peppermint so I'll try to be as objective as possible. First off, let me say that I respect every single one of the existing editors and their authors - so, there's actually no... direct competition.
My goal when I first started working on Peppermint (about 3 years ago), as anyone who has been in love with code since a little child (like me), was to create a Mac-native editor, which would look awesome, would support all possible syntaxes, with a built-in terminal, and super-fast extensibility. And that's what I did. Basically, the idea is a batteries-included no-need-to-leave-your-editor philosophy, combined with ease of use for beginners (no cryptic/hidden options, no nothing) and unlimited possibilities for those (like me) who want to endlessly... hack their editor, and tweak it as they go. :-)
P.S. If you want to get an idea of what I'm talking about (regarding extensibility), just have a look at the published plugins (they're open source, and anyone can add their own) -- I don't know how easier it could get! : https://github.com/osxpeppermint/peppermint-spec-plugins, or the API docs: http://osxpeppermint.com/documentation