Pretty sure you'll find 90% or more use Mach3, 10% or less use EMC/LinuxCNC. Both work just fine, but most people have Windows PCs and are not fluent in Linux, plus CAD/CAM choices in Linux are very limited. Mach3 is, arguably, easier to setup, and definitely much easier to customize. LinuxCNC can be brought up painlessly using a bootable CD (ISO image can be downloaded from LinuxCNC website), to see what it's like.
Regards,
Ray L.