Bubba-
Having watched some hardware solutions that looked good on paper come and then go, I now take a pretty conservative approach.
At this point the broadest and most stable performance with Mach is still the PP solution. The SmoothStepper is looking pretty good, but there is still some functionality it doesn't support, so I'd call it in "Alpha" form at this point, close to being in "Beta" when it will have implemented all the functionality of the PP Mach version.
If you are building your machine now, I would definitely go with the PP solution. I use a SoundLogic breakout board with mine and it has worked well, but there are other breakout boards that also give good results.
The good news about the SmoothStepper is that is appears to be pretty easy to move to it from a PP machine, and if it gets to the point where it is really rock solid and is in wide use by the Mach user base, including the Mach developers, then I'll move to it also.
Paul T.