ONE of the differences is that the frontlight can't be turned off without jailbreaking it. That's not much of a problem on the Voyage but it *does* make the PaperWhites somewhat more washed-out with less contrast in a bright room or in direct sun.
A jailbroken PaperWhite with the ability to toggle off the frontlight (KoReader offers this:
https://github.com/koreader/koreader/wiki) makes a big difference under these conditions.