The eTi is a dual heat exchanger system with direct fire titanium coils. It’s way more complicated than the MasterTemp and I don’t think the efficiency savings will justify the upfront cost and the expense of replacement parts over the life of the unit. Maybe if you were building some kind of commercial pool it would make sense. But fir a residential pool, it’s overkill.
If you believe the efficiency data then your looking at a 400,000 BTU/hr rated heater with a copper coil that gives you a usable 360,000 BTU/hr of heat versus a titanium system that gives you 95% efficiency or 380,000 BTU/hr of usable output heat. So you would be saving about 20,000 BTU/hr.
Most residential gas is priced in $/Therm. 100,000 BTUs = 1 Therm. So your savings would be about 0.2 Therms per hour. My gas costs roughly $1/therm. So the savings for my situation would be about 20¢/hr of use.
A MasterTemp 400 online pricing is about $2800. The eTI 400 looks to cost around $6,000.
($6000-$2800)/(20¢/hr) = 16,000 hours
So I would have to run the heater for 16,000 hours to break-even on the investment. Doesn’t sound like much but one rarely runs a heater for more than a few hours per week. Therefore the number of pool season the heater would need to run problem free would far outstrip the useful life of the heater.