Hi,
I suggest you to buy FPF40 as this model comes with auto mode which the unit itself comes with dust/odor sensor where it will automatically switch to higher air flow volume when it detected those unwanted particles in air.
Alternatively, you can consider to buy Panasonic as even it's not as famous as sharp in air purifier products, but rest assure that matsushita group is producing one of the best quality air purifier in the market. Open the unit and compare the filter quality then you will know what I trying to tell.