Thinking for a Change [TFAC] is a great introduction to the role of the the automatic body of habits (what we refer to as the 'robot') works and how to modify it.
Habit Hacking builds on the notion of the robot and provides a five step framework for breaking bad habits and building new ones. It's a more detailed and different approach than covered in Thinking for a Change.
For starters it's a video interview style rather than a workshop where Tom and Michael guide you through 30 modules on how to hack your habits. Also Habit Hacking is habit agnostic. It doesn't teach you a specific habit per se, but teaches you how to change (almost) any habit. So the emphasis is on habit change, building habit chains, leveraging tools for habit change, etc., introducing explicit patterns for change, much more than the scope covered by Thinking for a Change. However both training are complementary and bring something new and an important set of perspectives to behaviour change.
Habit Hacking shows you how to work with much greater precision and at the behaviour level than was possible to cover in TFAC. As someone who has been through TFAC, you'll be able to quickly build on what you've started there and target specific habits you'd like to break/form and then follow the step-by-step program and additional strategies taught inside Habit Hacking to do so.
We hope this will help you in making an informed decision as to what course/program best fit your needs right now.
Your can also check complete product information for Habit Hacking and Thinking for A Change.