One obvious difference I can see at a glance is that Maker.js doesn't support 3D models, while FluidCAD does. I assume Maker.js is a lower level library aimed at interfacing directly with CNC machines, while FluidCAD is focused on 3D design.
I used Slug at a previous job. It is an excellent, artfully crafted library; really the pinnacle of software engineering in my opinion. Thanks to the author for donating the algorithm to the public domain!