Water Heaters - tankless vs. 75 gallon tank waterheater

hi-
i will eventually need to replace two 40 gallon old natural gas water heaters for a two flat victorian and would like to consolidate to one water heater to save space-i live in san francisco.
not sure if i should go for a 75 gallon or a tankless water heater??
we are 6 people plus guests, two bathrooms, two dishwashers, two clothes washers & two dryers.

the work on the old water heaters was done poorly-improper venting ( into to a brick column, not outside!) and if i install a new 75 gallon it will probably have to be moved to a new location, at Homedepot i was told i should upgrade to a tankless water heater and that the installation costs would probably be about the same with all the upgrades, but at Lowe's they said installations on a tankless would run up to $2k.
any thoughts?
oh yeah-i need to install asap as the lower unit currently has no hot water-brr. thanks.

It sounds as though you're considering a single tankless for two separate households. You're probably going to need to get something rated at 200,000 BTU (or more!) to make sure two showers and a few other taps don't exceed its ability to heat the water. Pressure may be an issue. Fairly dividing the gas and water utilities on a system so conjoined may be impossible.

I'd strongly suggest you consider two units to replace the two tank water heaters, but, if not, you should definitely speak with a manufacturer-authorized dealer that specializes in tankless water heaters, and not the floor folks at a big box home improvement store.

thanks. that's the best concrete advice i've gotten so far. what about a shared 75 gallon tank water heater for both apartments? you're right our utility bill will have to be shared.