The attic should be properly vented regardless of how you choose to
cool the inside of the house. The best solution is a ridge vent with
adequate soffit vents. You also need to make sure that the soffit
vents are not blocked by insulation. It's very common for the attic
insulation to be installed improperly so that it's covering the soffit
vents and not allowing air to flow. There are plastic baffles
available at home centers that can be stapled to the underside of the
roof sheathing for a couple of feet near the soffit vents to keep the
air channel open.

A whole house fan can pull cooler outside air into the home very
effectively. However, I think they are most effective on days and
climates where you can get away with them without using the ac. If you
intend to use the AC during the day, then switch to the whole house fan
at night, I tend to doubt it will save much or be worth it. By the
time the outside air is cool enough to be of help, the AC load will
already be greatly reduced and the system should not be running much.
Also, the whole house fan will not deal with humidity, so you may be
replacing drier air with air that is more humid.