I'm a sainsburys lass. I did use tescos when I lived in Guildford as it was the nearest one to me but by God the quality of their own brands was awful! I think it depends what you want from your own money, I do want value but I want quality to0 and having tried most of the shops out there I always return to Sainsburys. I actually like their own brands more than some named stuff and I feel like what I get is not a waste of money because I like the quality.
That said, if finances allowed I'd absolutely go to M&S as it is pure food porn!