Keep going or try something new with my WebApp?

I'm currently in a situation where I am developing a Web App, and have it about 80% complete. There are starting to be concerns about it's viability as a business (no money has been spent on ads/testing yet). At the same time there is a new Web App idea that seems to have much better potential. Most sources seem to emphasize only working on one project at a time . Which project should be the focus?