Do I bake muffins in Convection or Traditional Bake mode?

It depends on how wet the batter is.

Because foods cooked in Convection have more moisture retention, I use Convection for most of my cooking; however, when it comes to baking there are times when traditional Bake actually results in a drier crumb. It all depends on how liquid the batter is. For example, I find I get better results baking Blueberry muffins in Traditional Bake because those blueberries just burst open and give off so much juice during baking, and if I bake them in Convection the muffins have a tendency to fall apart.