Any benefit to or negative for using one verses the other?  This is a dceision that doesn't have to be made right this instant, but I am starting to make some progress on my Hurco retrofit again, and its something I need to decide sooner rather than later.  I have a USB Smoostepper for the motion interface to the servo drivers, and everything else.  I was wondering if anybody had documented any issues or performance and stability issues with using the PS2 style interface for keyboard and mouse verse using the USB ports. 

The computer I plan for the big mill is a stable machine that has plenty of both types of ports.  It will get mounted inside one of the control cabinets, and have bulkhead connectors in the side of the cabinet for everything (monitor, thumb drive, etc) so I do need to do atleast some advance planning. 

If I have the choice I prefer PS2 but USB is often easier if the keyboard is a fair buit away from the computer as a Hub can be easily put into the control panel for all USB peripherals. USB will occasionally drop out for me on keyboards and touch screen but PS2 and Serial never do.
Hood

Total distance from PC to keyboard and trackball will be about 12' by wire.  There will be a bulkhead connector in line, and possibley a jumper/extension cable for distance.  

PS2 would manage that ok I think, cant remember the distance but I did have that on one machine. Problem with PS2 is you need a cable for each where USB you can have a hub locally to connect all peripherals so only one cable back.
Hood

I was also wondering if it might be an issue with using a USB motion controller. 

No problems with that, I use the USB SS on some of my machines and USB peripherals and they are fine together. As said the only issue I have had is occasional drop out of the keyboard/trackball and slightly more regular drop out of the touch screen but even the latter is only maybe half a dozen times a year and it is always at start up of the computer, never during use.
Hood

USB devices can be overcome by noise easier then PS2, it can be hard getting a motherboard with PS2 jacks though. You can get a two port PS2/USB converter that will handle your PS2 mouse and keyboard though.

I have a small florescent light above my work bench whose ballast is slowly going bad. If I turn it on/off all the USB  devices on my shop PC will cut out. The light was injecting some noise back on the mains that was making its way through the PC power supply and onto the 5V USB bus (had a brief 8VPP spike!) I added a line filter before the PC and solved most of the problem.

Getting rid of the source of the noise is usually the best option, but if you can't a PS2 keyboard might be a better choice than USB.