The difference is that they're two completely different ingredients and serve different purposes.
Butter is 81 grams fat per 100 grams - or about 80% fat.
Sour cream is only 20 grams fat per 100 grams - about 20%.
In a quick bread like banana bread, recipes often call for fat in the form of oil, not butter, which is almost pure fat. Either way, these recipes need to add moisture (much like a cake batter) and sour cream has lots of moisture and, additionally, adds a slight tang. So, recipes usually call for a combination of butter or oil and some sort of wet ingredient like sour cream, milk, yogurt or buttermilk.
The more appropriate substitution comparisons would be either:
From Baking Illustrated Banana Bread, page 23:
We still wanted more moisture in the bread, so we tried mixing in milk, buttermilk, sour cream, and plain yogurt. Sour cream added richness to our bread, but it also made for a heavy texture and an unattractive, pebbly crust. Milk added little flavor and created a slick crust. Buttermilk added a delightful tang, but yogurt let the banana flavor stand out. And because yogurt has more solids than buttermilk, it made for a somewhat more solid loaf, which we preferred.
As a note, their final recipe uses both melted butter and yogurt.