All I know is I have a H50 and it does exactly what I want it to do, my temps @ 3.8 on a Q6600 full load dont go above 50-55 after hours of prime, So tbh - no matter which you get there will be maybe a few deg diff, if that... so who cares - After looking @ the eco though it doesnt look as well made as the H50 but they are both not gonna last -aslong- as ppl will want them too because they are sealed units - unless you mod them etc.
Imo it really doesnt matter which one you buy, there wont be a great deal of diff - its personal pref and a few quid more. But I personally think the H50 is a better quality build than the Eco.
I don't know how you can say that the H50 is better built if you have never seen an ECO in the flesh! I would say the only noticable difference in quality between the units is that the H50 uses braided cables whereas the ECO doesn't. This is only likely to make a difference to the lifespan of the unit if you frequently move the unit from system to system - and even then there is only a small chance of the cable being damaged.