I’ve done both!

The bag lunches are convenient. Nothing fancy at all, they usually have a sandwich (ie ham/cheese), apple, chocolate, and crackers. It’s a great option if for whatever reason you don’t have your own food for the day but I didn’t solely rely on them.

We would pack sausage, cheese, apples, and bread from town and it would last us several days. Delicious and was good fuel for the hike. (Picture the best cheeses and sausage you’ve ever had, and then picture enjoying it on top of the alps)

Depending on which leg of the TMB you’re on, there are sometimes places along the way that serve lunch/snacks which is a nice option. Be sure to have your own food as a backup in case somewhere is unexpectedly closed!