I disagree with the Denner comments. I buy at both shops, Migros and Denner. Surely sometimes I even find really old stuff at Denner, but most of the time the fruit and veg is a lot cheaper and tastes just as good as Migros, for nearly half the price. I bought cherries, peaches and cherry tomatoes this week from Denner, and they were all tastier AND cheaper than from Migros. Same goes for "Pouletgeschnetzeltes", which is basically just raw frozen chicken strips. The ones from Denner are much better, tender and tasty, plus they are cheaper. The ones from Migros was terrible. So I would generalize too much about what shop has the better produce..... (just my 2 Rappen)
PS: I won't be using the 10% off from Migros today. I've got everything I need. Just another trick to make you consume more.