Both options have their pros and cons!
I will personally prefer legalization over dark economy. The main reason why bitcoin isn't seeing enough growth is because it has not yet been adopted and legalized by many governments. If we want to see real organic growth of bitcoin, it must be circulated within the economy. It is only possible when governments around the world legalizes it. We are still standing at a very nascent stage.
On the other hand, dark economy is not really people's choice! The common people really don't want to live under a constant fear of being caught. So if bitcoin remains a preferred choice for dark economy, common mass will stay out of it. It will not give bitcoin the desired amount of attention it deserves.
So my personal choice will be legalization, obviously!