Air Condition

Hi, would be greatly appreciated if someone could provide some advice on AC options for our brownstone. It is a 4 family home - owner's duplex and 3 rental units. We plan to install an AC system that could be used by all units, each with its own thermostat. A central system is not an option (I believe) as we are not planning to gut renovate the house and hence it would be difficult to put a duct system in place. Hence, I believe we only have 2 options, heat pumps or split systems. (don't know the difference either) Any advice on whether we have other options? Any advice on the pros and cons of each option? Thank you in advance.