I would recommend LinuxCNC with a MESA board(http://www.mesanet.com/). It's a little harder to get running than Mach3, but I haven't looked back since moving my router over to LCNC. The people at MESA provide excellent setup support and once you've set up a LinuxCNC machine once you can do the next one in ~15-20 minutes.

I use MoI with laser cutters, routers, CNC mills, 3D printers. I actually picked it up because Rhino was (at the time) very bad at outputting for 3D printing. MoI is such a joy to use that I worked it into my pipeline as the first step :)