Both live in the cloud. One is built around analysis.
Dabble and DraftProse agree on the basics of a modern writing tool: it should run in the browser, sync everywhere, hold your manuscript as a structure rather than one long file, and keep the editor quiet. Dabble built a strong plotting board, the Plot Grid, on top of that, for writers who track many threads.
DraftProse spends its extra weight on the Reader instead. The binder and editor are there, but the feature that does not exist elsewhere in the same form is whole-manuscript analysis that reports back without ever writing for you.