Apples and oranges really. Workflowy's features are quite a bit more advanced and last I heard, Matt Cutts uses it himself. I wrote https://zetabee.com/text for myself before Workflowy launched and it does a lot more than Google Tasks too. But Google Tasks is handy for short-quick-todos list users since it's part of Google apps. If you want something more complex, you will end up using Workflowy or better. Workflowy's target is not grandma-remember-the-milk types but rather more detailed project-management planners.