50 gallon vs 75 gallon water heater for an 8 bed, 3 bath SFH?

[Bruce Tieu]

The water heater at my house just went out and now tenants are complaining about there being no hot water. It was almost 19 years old and has a water capacity of 50 gallons. The first hour water rating is 80 gallons. So, it's time to replace the water heater ASAP.

Initially, the house was 4 bedrooms and 3 bathrooms with a private bath in the master bedroom. Since I am house hacking, I added 4 more rooms for a total of 8 bedrooms. Currently, there are 5 tenants living in the house. I plan to rent out two more bedrooms, so there will be a total of 8 people in the house including me. In total, 3 people would share the upstairs bathroom, and another 3 people would share the bottom bathroom. The person in the master room would just use their private bath. This is the current showering pattern of the tenants:

Tenant A doesn't have a specific time when taking showers, but takes 1 hot shower a day and it's around 20 minutes long.

Tenant B takes one hot shower a day and it depends at what time he comes home from work.

Tenant C takes one hot shower nightly at 9pm and it's around 30-40min.

Tenant D takes one hot shower in the morning for 10-15 minutes usually.

Tenant E takes one hot shower every other afternoon for 15 minutes.

Personally, I take cold showers. There is one washer and people don't use the dishwasher. There's a total of 4 sinks.

I talked to my plumber and the quote he gave me for replacing the water heater (parts and labor) with a new Rheem 50 gallon water heater with a 6 year warranty from Home Depot is about $1450. If I opt for the 12 year warranty, the total goes up to $1650. On the other hand, getting a Rheem 75 gallon water heater with a 6 year warranty from Home Depot would total $2250. 

Based on the shower schedule data, which water heater capacity should I go with? I'm thinking it's safer to go with the 75 gallon water heater to be safe, but it's an extra $800 and I don't know if it's worth going that high if the 50 gallon will suffice.

Thanks.