This is a long subject. The main difference between the two is, Environtech's products have a higher recovery rate because they are Gas Fired. Whereas Sets Products uses electrical. For energy efficiency the gas powererd units are better. For gpm flow the Gas units are better. For temperature ranges the gas units are better. The gas units can heat the water from a lower inlet temperature 40 degrees to 140, compared to the Sets units 50 to 140. So alot depends on your inlet temperature and the availability of gas.
Although I'm not a fan of a instant hot water heater in a retro-fit situation. I do like them for New Home installations. Piping needs to be sized properly to reduce any possible restrictions to flow.
My only questions would be, will you save enough in the life span of the water heater to pay the additional installation costs and the higher replacement costs over a standard tank type water heater. You definately will save costs over a 90 plus water heater, because the installation costs and the replacement costs are the same. I would hope the cost difference will be enough to pay the higher price, but they are such a new factor here in the US we do not have the studies yet.