The UC300ETH and UC400ETH will work with both Mach3 and UCCNC, and if Warp 9's past record holds true, they will have Mach4 plugins before the USB Smoothstepper.

You could replace the SS with a UC300ETH, and not have to make any other changes to your system. This gives you the option of trying UCCNC along with Mach3, and even Mach4 when they get a plugin finished for it. (So far, CNC Drive only has a UC100 plugin for Mach4).
This gives you the most options, imo.

You could also just replace the USB SS with an ESS, which should work much better. Or as mentioned, move to CS Labs, or Vital Systems boards, which are probably the best Mach3/Mach4 options out there, at the high end of the price range.