It depends on your timescales. Right now Flash is far more widespread than HTML5, mainly due to browser support (like it or not, until IE9 is released and widely adopted, this will remain the case).
However, I think over time HTML5/Canvas will become the dominant technology — at least for things like video, animations and simple interactivity. I suspect that Flash (and Silverlight) will adapt and survive, but will have more of a niche market.
I don't think you'll waste your time learning Flash, but if I were you I would look at HTML5 first. There will be far fewer experts in HTML5 in the near future, and more demand for their services, so I would see it as the smart thing to do.