They are probably both right.
Technically, with slipons and the Screamin' Eagle air cleaner, all you need is the Stage 1 download. These bike run lean as they are programmed at the factory to run at the EPA legal air fuel ratio of 14.7:1. As a result, they tend to run hot
Slipons alone will most likely have little or no impact on the bikes performance or the A/F ratio. Adding the S/E air cleaner will definitely lean out the A/F ratio and cause the engine to run even hotter, possibly eventually damaging the engine, unless something is done to modify the settings in the ECM. With slipons and the S/E air cleaner, the Stage 1 download (a one-time pre-determined by the factory adjustment to the ECM) supposedly brings the A/F ratio back to 14.7. Your bike will run somewhat stronger, but not any cooler. The Stage 1 download costs $150 plus about 1/2 hour of labor to install.
Using a SERT instead of the Stage 1 download allows a lot of adjustment to the ECM settings. It requires a significant amount of dyno time (approxinmately 3-4 hours) by a skilled operator to adjust the ECM settings so that your bike will run better and cooler. The SERT costs several hundred dollars (about $300-400), plus 3-4 hours of labor.
There are many other options for making your bike run cooler and better, and there are twice as many opinions as to which is the best way to go.
I am sure Glider will chime in on this, as he has his own recommendation.