Apart from different publishers, different authors, and different writing styles?
If you don't mind an outsiders' viewpoint, Head First is all about approaching a domain of knowledge that you are either entirely new to, or which you may not yet have fully grasped, or where the existing literature may make things needlessly cryptic to the novice. The series has a particular tone of 'voice' and uses very specific pedagogical concepts to impart the lessons.
The Head First series is more about jumping into and understanding something completely new, whether that be HTML or SQL or Design
Patterns, or even Chemistry for that matter.
There doesn't really seem to be a 'refactoring...' series, though several of the Addison-Wesley Signature Series include the
word in the title.
Refactoring is a very specific discipline, so while a book about Refactoring may be introductory, it will most definitely be centered around this discipline and how to apply it in a specific context or using a specific language). It won't be written in a specific educational style (Lions, and Tigers, and Bears - oh my), and I very much doubt that we will be seeing a 'Refactoring Molecular Biology - The Incremental Improvement of Genetic Code under Controlled Conditions and Which Do Not Impact The Observed Behavior of the Individual' anytime soon.
I wouldn't be surprised if we see a Head First Genetics, though.