I've never bought either before, but after spending Christmas Day in the kitchen last year, this year I'm buying time.
The nearest M&S is 15 miles away in a busy town. My local Sainsbury's is a stone's throw. I could make M&S work at a push as we have a day out on the 22nd and could collect the stuff on the way home, but the desserts might take a battering.
So, is Sainsbury's pre-prepared Christmas food on a par, or does it taste like ready-meal fare in comparison?