Workflowy is strictly an outliner. It's like Gopher--hierarchical, unlike the Web, which is a graph.

Feature-wise, Roam is more like a graph. You can really easily link to other concepts, and rename pages (and everything renames). It also has a page generated daily, for things you want to write down.

Feeling wise, you get to write things and collect them and organize them later. I think it's more condusive to how people think and research. You might have a piece of data, but you're not sure where to put it yet. Most other note taking systems forces you to categorize first.