Interesting question with multiple answers. Probably not what you wanted to hear.
Heater in parallel have specific plumbing issues which need to be designed in from the start. The supply lines and discharge lines (up to the hot water main supply line) absolutely need to be hydraulically the same. Otherwise one unit will work harder and more often because it is carrying the load.
In series, the first heater is just a pre-heater that takes the water up 20 to 40 degrees with the second heater making up the remaining temperature. Many of these units will also have constant recirculation (on larger installs) and you will find that is somewhat cheap to run this way.
Don't confuse no cost or little cost with major cost...you still need to heat 80 to 120 gallons of water and keep it hot. It just costs less on large applications of central water heater banks (with tempering tanks) in condos or apartment buildings.
Something else for you to consider is that units are usually placed side by side with common venting also considered. You will need to look at your current vent sizing and location.
Have fun and ask if more questions.