Supermarkets do vary in the types of people that they employ and the types of customer that they have. Waitrose and M & S tend to have more middle class customers and also more middle class staff. You will need to decide which you think that you may fit in better with. Imagine whether you would be happy talking to the types of people that shop in the different supermarkets and that may give you a better idea as to which would suit you better with regards to employment.