Advice on new electric HWH.

I currently have an oil fired, I think it's called a tankless coil, furnace, that also heats my hot water baseboard heating.
For economic reasons I am going to install an electric cylinder.
Is there any benefit in paying approx $1500 (Canadian) for a Rheem Marathon, lifetime warranty unit, over $600 - $700 for a standard unit with 8 - 10 year warranty.

Also what is the recommended size unit for a family of 4, who use the shower, not generally the bath.

Typically houses that use a Geothermal are all electric. An 80 gallon electric will catch more of the free hot water off the desup.