All sockets should take two 4mm wires but MK claim two 6mm wires. As to quality only when max is drawn does one really test the socket and the normally failure is due to over heating. If course metal conducts heat better than plastic so metal faced sockets tend to last better and don't show discolouring even if it happens. However impact damage can be hidden.
I like MK as the holes are slightly bigger so easy to work with and the grid range do seem better made than others but really it is impossible to tell. As an electrician we very rarely get to know why a socket failed. People are loathed to admit the pulled on the vacuum cleaner cord or plugged in a 20A electric welder only the users really know what they have done.
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