Besides all other things already mentioned, the OpenGL engine has been rewritten for AC19 so the same model will be running much more smoothly in the 3D Window in AC19 than in AC18.
So AC19 is the better choice.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD 2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28
If you ask me i would say archicad 19 is performance update and you can live with AC18 without any problems...If you have modeling problems in AC18 you will have them in AC19 too, so don't worry, workflow is the same !
Other architects who works in AC19 will not be able to do anything you can't do in AC18.
Get yourself AC18 and SSD...especially because AC18 is more polished then AC19 (bugs)