The separate heat pump offers much greater serviceability than the one piece units. Stratification is just not that different between the two units. Given the longer recovery times of any HPWH, you pretty much want the whole tank hot all the time. The lower cost of the all in one units is compelling. A $300 unit separate unit that is one generation back is even more compelling. I hope they all last a long time. Between the two, I would prefer to have something that I can work with that is less expensive.