Welcome to the forum.
Can't make any recommendation on a heater because I don't know details of that heater. However, I would put the gas heater ahead of the electric. With the electric heateer you get 100% of the energy converted to heat because the heat source, the resistance heater, is in the circulating water. With a gas heater you want the maximum temperature differential on the heat exchanger to get maximum efficiency. If you had the gas after the electric you would be pre-heating the water and reducing this differential and thus the efficiency. Probably not much of a difference either way but every little bit of efficiency gain helps. If anyone sees anything wrong with this thinking please speak up.
Al
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