"Any idiot can come up with a perfect plan for war as long as he's not actually doing it." --Churchill
In one of my classes with former Mayor Riordan, he presented these two quotes to us in connection with his "axioms of leadership."
Voltaire's quote is definitely at the heart of entrepreneurship: the importance of "getting on with it" and making decisions. This ties in to Riordan's "courage" axiom of leadership.
Churchill's quote goes one level deeper into this in pointing out that it's much easier to be an external party considering what would be best for others to do rather than having to actually do it yourself (especially when it's something as difficult and risky as going to war -- or launching a company). That's why many people write about it instead of doing it.
An interesting way to combine the two quotes is to realize that when you're making a plan for yourself, you can't aim for perfection -- goodness and timeliness matter most.