Rarely will a sealed foam insulated attic deviate more than 15 degrees from the living space temperatures below it.
Attic vs crawl space.
My vote would be the crawl space if there is sufficient room and easy access for service.
You can expand it just enough to accommodate the ducts.
As a great deal of heat can escape through the attic of a house it is common for this area to be insulated.
In a crawlspace you have to worry about critters and duct losses.
In the attic you have worry about heat gain and duct losses.
An attic crawl space is an area on the top level of a house that is not tall enough to be used as a living area and is often unfinished.
This is a big deal and has the potential to add significant cost to air sealing insulating crawl space encapsulation and all those things we do in confined spaces.
The attic would be fine if the equipment and duct work were installed in conditioned space.
Insulate and air seal the floor with open cell foam put ductwork and mechanicals in the attic install a radiant barrier and mastic seal the ducts.
I vote for the attic.
So when we have a choice we prefer hvac systems located under the house.
However i have two questions if i want to centrally air condition my entire house.
If you have vaulted ceiling then you will need to install some registers near the ceiling on one of the walls.
It s easy to see your point about eliminating ductwork from the attic.
Compare that with the daily 140 degree temperatures found in vented attics.
Expanding the conditioned space doesn t require you to condition an entire attic or crawl space.
Strap ducts out of insulation with 3 ductstraps per specifications.
If there were a way to eliminate the possibility of critters i would prefer the cooler location for ducts.
With new construction i would look for a way to have them in conditioned space.
Seal supply boxes with mastic.
A two room addition on the 1st floor has limited attic space and a 4 crawl space.
O is air conditioning the 1st floor effective with floor venting if i run ductwork in the basement.