Online classes need a reasonable internet connection, a quiet space and a parent who keeps the routine going; without that they drift. Some children also miss the social side and find a screen less engaging than a room full of friends.
Madrasas vary enormously. Some are excellent. Others have large classes, long sessions late in the day, limited correction of Tajweed, and little communication with parents about what was covered. Travel time adds up too, especially with more than one child.
Cost is worth comparing fairly as well. A madrasa is usually cheaper per hour, but the useful measure is how much individual reading and correction your child gets for the money. An hour in a large group and thirty minutes one-to-one are not the same thing, in either direction.