Any idea what resolution the free MAXWELL limits you to? I was going to do Kerkythea, but it seems to no longer have a website to download from.
With the free version maximum image resolution is 800 x 800. With the licensed version maximum resolution is full HD (1920x1080) and it includes a production render engine - which means that it is equipped to deal with more complex lighting. Of course there are other rendering engines for SketchUp that will run on Mac. I also use an iMac.
For myself I would like to have tried Blender but I found it hard to get started with it. SketchUp was a (relative) breeze to get started with, but admittedly it doesn't work like Blender.