Alpha-N all the way. There are those that do not like Alpha-N systems, but with proper mapping, processor speed and enough resolution (what the early H-D system lacked), Alpha-N works fine, even better with higher degrees of engine modification than a "speed density" system. Speed density relies heavily on the MAP sensor input, and can be confused with longer duration camshafts and an uneven engine such as the Harley. If you research it, you'll find that most all performance EFI systems are Alpha-N based, regardless of application.
This is not to say the factory speed density system can't be made to perform well; it will require a skilled tuner with in-depth experience with the race tuner software some time on a dyno to build a map for the application. Which brings us back to the "proper mapping" I mentioned earlier. With the ThunderMax's growing library of base maps, and now the addition of the Auto-Tune wide band closed loop option, the need for extensive (expensive?) dyno time is minimized, even eliminated.
There are those that will argue these points endlessly. This is why we offer the buy-it-back guarantee. The best way to decide is to try it! Thanks, JK