They are two components that don't work well in unison. You are often forced to pick one and compromise the other; Complete the project quickly or make sure everything is perfect about it. The understanding of this principle is actually an ideal metaphor for life in general. Everything we do requires the yin and the yang - the ultimate balancing act. It's the high risk equals high reward methodology that so many professionals abide by on a daily basis.
So then what yields more success? Entering a market quicker than everyone else or creating a superior product after everyone else?
The answer is neither.
The sweet spot, again, just like everything else in life, is finding the right balance. Keep both extremes in mind and move quickly, but not erratically. Make mistakes, but cover up the most noticeable ones.
At the end of the day, you'll have more time to refine the output with all of the feedback you'll be receiving from users after they are exposed to it. It's going to be an ongoing process to perfection regardless. Realize that ahead of time.
If you don't end up being at least a little ashamed of what you put out, you moved too slowly.