I didnt try the Tuneboy on a SD, but here is some facts.
Tuneboy offers a few more things than the PCIII. With the PCIII you can create a custom air/fuel map.
With Tuneboy you can also control/adjust ignition and move the rev. limiter. Some bikes can easily rev. higher than standard. One of my friends tuned a Triumph, and moved the rev limiter 1000 rpm up, because the bike still was producing power.
My best guess is that you dont need the extra features the Tuneboy offers. Also Tuneboy is a little more tricky to work with (if it is your first time, no problem for a Tuneboy pro).
Working with the PCIII is pretty much straight down the road (if you have a Dyno..... ), and there is many Dyno experts around, some better than others.