I prefer them in the attic. This is a personal preference, not an installation fact. We move lots of home furnaces from a hallway closet to the attic. Here are some the advantages to moving your furnace from the hallway closet to the attic:
They are quieter: Furnaces suspended from the rather joist do not allow vibrations to enter the ceiling. The noise emanating from the return air register under the furnace is dampend ducting and an indirect transition to a ceiling can or wall chase.
They can heat more evenly: If installed correctly with multiple return air registers the attic installed furnace can help to even out temperatures throughout the home by pulling stagnant air from different parts of the home.
Water damage is less likely: If installed correctly the furnace and evaporator coil will have a pan with a safety shut off embedded in it. If the condensation line does stop up it first drains into a secondary line that terminates above a window or door. If this is ignored and it plugs up, the pan under the furnace and evaporator coil will fill up and turn the furnace off before damage results. This provides one more layer of safety from property damage.
The filter is more likely to be serviced: In most attic installations the filters (primary) are located in a common area such as a hallway. When you are walking down the hallway one direction of travel will allow you to view the surface of the filter that gets dirty. The constant reminder enables these to be a closer to “Top of Mind” and be serviced more regularly.
Moving your furnace gets you an extra closet: Most homeowners utilize the existing closet that opens up when we move the furnace to the attic as a linen closet. Often this closet abuts a bathroom and the size of the bathroom can be expanded to encompass the newly freed area.
They are easier to service: The hallway doesn’t get to 140 degrees in the summer making air conditioning maintenance much easier for the service technician and cheaper for the homeowner.
You Don’t need a ladder for filter changes: The furnace filter is typically located in the bottom compartment of the furnace and if you have an air cleaner it is attached to the bottom of the furnace making for easy cleaning and easy access.
This is a preference that you need to decide if you want. Both furnaces will heat and cool your home and installing a furnace in an attic or hallway will not make any significant difference in efficiency.