Tankless vs traditional water heater for multi family

[Kip Cline]

I'm considering buying a 2 story 4 unit building. Each is a 2 bed 1 bath. It has common water and gas but separate electric meters. There are 5 meters one for each unit and a common area one. Each has 4 20 amp plug fuses. The building needs a water heater. I'm wondering what would be my best option. 

1) traditional gas water heater in the common basement. If so what size/how many?

2) tankless electric water heaters in each unit. This would shift some electric cost to the tenants. Would I have to upgrade the electrical? Cost? 

Any help would be appreciated.