The EC holds more water. this is important on steam systems. Newer high efficiency steam boilers with low water content. Tend to produce steam too quick, and short cycle. Causing high heating bills. And very uneven heating of the house.
While there is a price difference between the 2 boilers. Good chance most of the install price difference is in the install its self.
A poor install of a steam boiler, has far reaching repercussions.