Roger - The integrated controller (manufactured by Auberins) on the D models eliminates the temperature swings common on the analog smokers, which are not a hindrance to good food. On the analog, the controller cycles on and off slowly, allowing the temperature to exceed the set temp by maybe 15 to 20 degrees; then cycles off till the temp drops below the set temp by an equal amount. Then it cycles on again. The D model controller cycles constantly, keeping the temp within a couple of degrees of your set temp. In the end, the resulting Q is the same. The D model also has a built in meat probe and the ability to cook at temperatures higher than 250 degrees.
If the latest in electronics are not needed, get the 3 for the extra real estate. Later on if you want you can add the stand alone Auber controller.