Personally, I lean towards the Vaillant, as I think they are super quiet and heat water quickly.
The storage ability is relatively new concept to allow hot water to be available regardless of the boiler state. In the winter when the heating is on, then you would probably not notice any difference with the hot supply. In the the summer say when the boiler is totally off, with a standard boiler, the primary has to be heated before the DHW which results in an extra delay before the hot reaches the tap.
Now with a good combi, this delay is minimal anyway and to most people is not a significant factor.
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