The sass on this thread is hilarious. BUT.... I like what @Otters said. Smol bread vs smol cake. (It felt appropriate to use the smol snek spelling here.) Banana bread = banana muffin but not banana cupcake.
I'd say they're two different things. Cupcakes tend to be made with self-raising flour, are smaller and decorated with icing to make them sweet. Muffins tend to be made with plain flour + baking powder, are bigger + denser and have things inside like chocolate chips.
Muffins can be enjoyed at any time of the day without judgement, however I personally enjoy cupcakes more. Unless it's a really sweet dessert muffin. Then I'd go with muffins. Can we have a "both, but they're individual small pastries and deserve to be recognized as such" option?