Consider the Fitted the Boxster of our lineup. Not as tight as a Miata, but definitely for those who like things more tapered. Expect narrower openings and bit of a slimmer fit, with a softer fabric.
The Heavy Duty on the other hand, is more like a Corvette Convertible. Generous openings and a boxier fit around the shoulders, with a thicker and slightly heavier fabric.
Heavy Duty tees also use a different color lineup, so consult the product page of interest to confirm the shirt shade first.