I have a large Bradford-White which is a high efficiency electric unit somewhat like the Marathon I believe. My geothermal system has a loop in it that preheats the well water before it gets to the hot water heater. The system works very well. Very low cost of operation. Always plenty of hot water even with kids and grandkids visiting, washing dishes, laundry and showering.

On hot water systems there is a tradeoff between initial cost and long term savings. If its for a place you are likely to be living for a long time, spending a little more up front can have a large payback over time. My electric coop had a rebate for installing an energy efficient model which helped with the initial cost.

Jim