I own a small motel that has two 40 gallon water heaters hooked up consecutively. They are about 20 years old. The furnace (hydronic heat) is hooked up to the water heaters as well and preheats the water before it goes to the 40 gallon tanks. I would like to replace the water heaters and was wondering would it be better to just get a 80 gallon heater vs the two 40 gallon water heaters.

Well depending on where the hotel is and what your using for utilities you also have a third option. Don't use a tank of any kind. That way your not heating water just to have hot water. Also then your patrons can take a long shower and not run out. Im not tryng to spam you just passing info for you to consider as an option for your situation.

Rinnai. America's Leading Tankless Water Heaters: Energy Saving, Forever Hotwater

There are many companies out there that do the same thing. Might be worth investigating.