they work well together.
If you need cad functionality then you will need rhino, to deal with DXF, DWG etc.
Plus rhino is pretty decent at 2d drafting, if you are familiar with Acad.
If you want to export LWO meshes, you should also get moi, moi's meshing is much superior.
Also, the workflow is imo much superior for a lot of modelling tasks. Rhino has a bigger toolset, but for 90% of modelling tasks, moi is faster.
You can work with both together, it is just a matter of copy/paste from one window to the other.