HardOps is a toolkit that encompasses more than just modelling. There are tools pertaining to rendering, mirroring, inserts and even sculpting tools. This is the core of my workflow in Blender and is recommended for anyone doing Hard Surface or just wants to use Blender much more efficiently.
Boxcutter is just a cutter. However, by just a cutter we mean this tool is focused on being the fastest and easy to use 3d view cut tool. If you love Hard Ops then Boxcutter is just another piece of the workflow. Drawing cuts and having real-time feedback is truly the next level of the workflow.