Hey Gang,
  I Have a  Bridgeport Series1 BOSS 5 Mill.  It's still all original but the "computer" is dead.   I would like to retrofit this unit with Mach3, some Slo-Syn SS2000D6 drivers for the stepper motors, and then use a Mac Mini to run it all.  I'm trying to keep windows computers out of our home.   Using a Mac Mini, I must use the USB output rather than parallel port.  This leads me to wander weather or not I need a motion controller or break out board?  Also do the Slo-Syn drivers need a motion controller or just a standard break out board?  I also really don't know that much about the break out boards and see them from 10 bucks on Fee-Bay to $79 at what seem like much more trustworthy sites.

 Which should I use motion controller or BOB?  AND  Is there one that will work better using Apple OSX ?

Ok, guess I was barking up the wrong tree.   I did some more poking around the net and could find no instance of someone running Mach3 on OSX.  I had emailed Mach tech support and was told you could run it on a "Mac" but they must have been assuming I meant through parallels or boot camp.  This would defeat the purpose.

So I'm now going to bite the bullet and just get a PC to run the mill.  With that said,  I'd still like some suggestions on a break out board or motion controller.   Should I be looking to run  through a parallel port or USB?  I assume running through a parallel port is easiest and would just require a BOB as long as that also fits the Slo-Syn drivers needs.

A parallel port can be the easiest, but you won't know until you try it. The issue is that parallel ports don't work the same from PC to PC. Mach3 will run very well on some PC's, and not very well on others.
You're also limited to 32bit OS's when using the parallel port.

Because of this, it's probably easier to get a motion controller. Some require a breakout board, and some have breakout  board functionality integrated into them.

The cheapest option is probably a UC100 and a basic breakout board like the C10, unless you need more integrated features.

Thanks a ton Gerry,

I've looked into both the UC100 and C10 and think I under stand the set up.   I think it should fit the bill.   I'm not familiar with what integrated features are. Things like e-stop, spindle break, and such? 

I would need signals for: ebrake on & off,  spindle speed up & down, xyz limits, and estop.    I've already seen the limit and estop circuts on the C10. 

I'm not familiar with what integrated features are.

Things like spindle speed controls, relays, charge pumps,...

The C10 is just a basic breakout board. The more expensive ones often have a lot of built in features that the C10 can't do.