Hi Brian,
The Smooth Stepper is an ideal solution for using a laptop with Mach3, I have used an SS for a year or so now and together with many others, I can only say how good it is.
The FSE plugin card is another story and I have not yet herd any reports of it working well with Mach3 despite the FSE claims and as their forum is so highly moderated, it is impossible to obtain a truly objective opinion on its operation or ease of use from any others there.
If the price of the card is reasonable then I think you should go for it but beware that the makers claims may well exceed its true performance.
Tweakie.