Regarding Vader's line about the power of the force being greater than the Death Star (ability to destroy a planet): I believe this to be Lucas' way of foreshadowing the eventual destruction of the Death Star at the hands of Luke Skywalker, as he uses the "power of the force" to prove Vader correct.
See, Vader isn't saying "Mine's bigger than yours!" He's saying that, yes, planets are big and all that, and if you can destroy one, that's great and all, but the force is the energy of all living things, and nothing can ever be greater than that. That's sorta the point of the movie, that the power we living beings possess is greater than whatever "technological terror" we may face, because we can affect change through our will, and that's what Luke does.
There's more to say, but it's late. But I will say Vader's line probably contains the last subtlety Lucas ever wrote. That's more of a lament than a slam.