I would go with the ceiling, because you will have fewer elbows in the airflow. Elbows decrease the strength of the airflow.

It depends, as you noted; heating is best with discharge at the floor level, A/C at the top of walls or in ceiling. The two different positions are for mixing the heated or cooled air better. I have both in my house upstairs is ceiling or upper wall and down stairs is out of the floor. Reason is to minimize the duct work lengths. The air handler is in attic over the ceiling on second floor, air handler is outside the house on first floor.
There are two zones and two heating and A/C units.

Under the floor is better. The heating system will lose a bit of heat in the winter which will slightly heat your crawlspace. Heat rises so your floors will get this extra little bit of heat. In the summer, your attic is heated excessively but the sun’s radiance so your A/C will have to work extra hard to remove this extra heat. The crawlspace is already cool so you save money because your A/C doesn’t have to work so hard or excessively. A win-win!