The process is the same for either manufacturer and the opening size is identical. The difference between the units will be the control units and thermostat, and whether you are installing a heat pump versus a purely an AC unit.
As mentioned above, getting the old unit off and the new unit up onto the roof is the most arduous part. The exterior roof units are roughly 100-110 pounds. You can rent a lift from a local tool rental company. I recommend asking for a "duct lift" with a 10 foot lift capability. These lifts can be transported in your pickup and be used/maneuvered by a single person. Around here, they rent for <$100 day and a half day is an option for around $50.