Trello recently implemented power-ups, that basically foil most of the advantages of Zenkit.

My personal rant: I wish Trello would take better care of its api. The REST endpoints all work great, but examples are very, very basic and languages other than javascript are basically ignored.

True, however you're limited to using only one powerup a free user, or 3 as a gold member. Things that count as power ups in Trello, like custom fields & the calendar, are standard in Zenkit and available to all users... Zenkit will also release an API to enable connections to other apps, just as Trello does.