This question assumes that sectionalism and slavery were not related. In my view, there is no way to separate these two. Much of the sectionalism was caused by the issue of slavery. Therefore, if I had to choose, I would say that slavery was more important because it helped to cause sectionalism.
Sectionalism can be defined as being more loyal to a region than to the country as a whole. There were a number of reasons for sectionalism, but the sectionalism that the US experienced was caused by slavery more than by any other factor. The major issue that divided the two sections was slavery. It was not that the North really wanted to abolish slavery. Instead, the North wanted to keep it contained within the South rather than letting it spread. The South, meanwhile, felt that slavery had to be allowed to spread. Otherwise, they believed, the free states would come to dominate the country and would abolish slavery. This was the main thing that set the two sections apart. It is true that the North and South had different kinds of economies. It is true that their societal values were somewhat different. But these differences were caused mainly by slavery. For this reason, I would argue that slavery was more important than sectionalism, even though they were closely related.
Secession was the immediate cause of the war. But we have to ask why the South wanted to secede and that is where things like slavery and sectionalism come in.