I finished building my 2nd CNC machine but my first using Mach3.  It's running well except that it occasionally misses a step and then my feelings get hurt (if I'm not quick enough on the E-Stop).

I'm currently going straight from parallel port to gecko drives to motors.  

If I slow things down, it works okay and I can optimize the computer (it's dedicated to the CNC) but I don't fully like either of these solutions.  So I'm looking for a stepper controller to offload the motor controlling.

After reading a bunch of posts, I've generally narrowed it down to:
ESS- Ethernet Smooth Stepper http://www.cncrouterparts.com/ethernet-smoothstepper-p-229.html
or
XHC-MK4 from Automation Technologies http://www.automationtechnologiesinc.com/products-page/breakout-boards/cnc-mach3-usb-card-xhc-mk6-4-axis-motion-control-card-usb2-0-interface-wshell

They're basically the same price and appear similar in features.  I've seen a few posts on each individually but no single thread with opinions weighed in on the two.  So I thought I'd get opinions from those that have been there before.

Thanks,
Hylan

The major difference between the two boards that you provided a link to is that one is USB and one Ethernet. I had a lot of issues with the USB version of the Smoothstepper. These were due to electrical noise from the spindle VFD. All of these problems went away when I changed to the ESS, using Ethernet instead of USB.
Re your missed steps, they may not be addressed by changing the motion controller. Missed steps are usually caused by the machine being on the borderline of the driving force that the steppers, drivers and power supply can produce.
So while an improved motion controller would be good, I'd look for your immediate problem of missed steps elsewhere.

Cheers
Dave

If I slow things down, it works okay and I can optimize the computer (it's dedicated to the CNC) but I don't fully like either of these solutions.

These two fixes fix two different issues.

If slowing things down fixes your problems, than that usually means that you don't have enough power to move faster. This can be caused by both too small motors, and also from motors that are too large. Often, large motors have high inductance which limits the rpm they can spin at.
Using a motion controller like the ESS will do nothing for you if the motors (and other drive components) are the problem.

If the PC is the cause of the problem, then the motion controller may make difference.
But you might want to do a little optimizing before spending $200. There's really very little that you can do to make a difference as far as "optimizing" the PC for Mach3. The biggest thing is to make sure the PC is running in Standard mode, rather than ACPI mode.

Great feedback.  My cnc is definitely not underpowered.  However, it is consistently my Z axis that misses steps which is my highest revs per inch axis.  Therefore, it does make sense that it might be trying to spin to fast relative to my other axis.  My X axis has two 960 oz in motors, Y has one 960 oz in motor and Z is 1 640 oz in motor.  I need to look at my settings and slow my Z rapid down.

Thanks,
Hylan

I finally made it into the shop to check my settings. My Z axis is 20000 steps per inch. I'll go back and do the math to see what my max speed should be.

Thanks again,
Hylan