I have done both. DC1 used to have a "supper" of cold food, like falafel, cous cous, crudités etc., as she'd eaten a full hot meal at school. (Such a big meal that the teachers had to ask her to leave some food for the other children). When DC2 went onto solids, DC1 started having a hot dinner again, as we were cooking for our youngest, and so might as well have cooked for our eldest, too.
DC2 has just started school and so both of them are now having cold suppers. Only this time, they have toast or a sandwich and some fruit etc. Mainly because they both complained about the cous cous, pasta salads etc., but also because it means I don't need to go to the supermarket for fresh food every few days. Having a cold supper has taught my DC1 not to overeat, so it's been good for her, and it means DC2 can get to bed early, which is necessary, as nursery means there is no longer time for an afternoon nap.
That said, my DC are in private school, and the food on offer (although bland to cater to childish tastes, as opposed to the adult food we serve at home) is decent. There is a hot meal covering all good groups, with a buffet of veg, a salad bar, with cheese, cold proteins, bread and butter, fruit and dessert. The school also give snacks in the afternoon, and then more again at after-school clubs.
If the food was horrible, I'd cook a meal, but they both enjoy their school lunches so I think a sandwich or whatever is fine in the evening.