They're all OK, but I'd choose the Vokera for the simple reason that my Vokera Linea 28 is now six years old and has never missed a beat...yet!
They will all have things that go wrong with them eventually, and all of these are good makes, so I wouldn't get too hung up about your choice. I think I'd go Vokera,Vaillant, Worcester in priority if I were pushed, but there's nothing in it really.
Most important is the installation, making sure the system is thoroughly cleansed and protected.
I would have to say Valliant as I have an 828 and have never had a moments bother nor from the ones I have fitted.
The divertor problem was quickley identified by the manufacturer so if you are buying new that should bother you.
By the way from my knowledge the Ecomax has now been uperseeded by the Ecotec which unfortunatly does not have the build quality (ie plastic instead of brass)but you might find an ecomax in stock somewhere
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