The COMM AMEL checkride is mostly VFR (meaning visual) while the entire ATP AMEL checkride is IFR (read as under the hood) and long. The flight portion of my ATP checkride was almost 2 hours with maybe 0.2 without the hood on. Make sure your instrument skills are sharp before you start prepping for the ATP checkride. You will also need to know your aircraft systems very thoroughly for the ATP checkride.