Multi-Family Water Heater Question

[Kyle Lecy]

I currently have a 23 year old water heater in an 8 unit building. The inlet and outlet areas on the water heater are corroding, and when last checked, appear to have water pooled around them. I would rather have it replaced before a full blown leak occurs. 

Looking online for a new water, I'm not sure of what to go with. It currently runs on natural gas. Should I go with a 98 gallon heater or will a 75 gallon suffice?