I believe the main advantage of a cruiser is riding comfort. The rider sits in a very upright position, and not as high. They also tend to come with fairly wide seats. They have a certain style. They come in a broad range of sizes for every size rider. They are typically in a price class for casual riders.
The main disadvantage is that they don't make particularly efficient use of a rider's power. They also tend to be a bit on the heavier side. You won't be racing a cruiser, and the exercise benefits will be more toward lower intensity.
The 7D has seven speeds (the most in Trek's Electra line), which accommodates a broader range of hill climbing and cruising speed. They also have hand brakes, to which you may be more accustomed.