I recently added a 2nd floor to a 1500sf ranch in the NE. The AC has been relocated to the attic above the new 2nd floor, but the return has not been connected, and some of the supply ducts still need to be run.

The 1st floor is basically one big flowing room - no walls per se - there are supply ducts on the perimeter ceilings from the attic AC.
There is a 400sf open to below area w/balcony/catwalk on the 2nd floor (over living room/starway/foyer) with supply ducts. There are 4 bedrooms with undercut doors and a supply duct in each.

There is a 24x36 shaft going from the attic to the ceiling of the first floor - to be used as a return if needed.

My question is this. Where should I locate the return?

a. In the ceiling of the open to below space, utilizing it as a "funnel" to suck all the hot are from the first floor up?

b. On the first floor, allowing the cool air from the 2nd floor rooms and open to below to fall and get sucked back to the attic through the shaft? (theoretically creating great circulation)

I know that going with option a. will force the AC to take hotter air and work harder to cool it off. But it will at least evacuate that hot air from collecting on the 2nd floor - probably creating an over all stable temperature in the whole house.

With option B, since the return will be sucking up the cooler air from the 1st floor, the AC will likely have to not work as hard, and I should get decent circulation from the 2nd floor supply ducts dropping cool air down to the 1st floor.

Any suggestions?