I have an addiction to ereaders.
If you don't care about waterproof (which I don't think you do), from your list of possibilities, if it were me, I would get a Voyage first, because it is STILL the most clear text of any of the ereaders I have; then a Clara HD, because it's so light and easy to use--and it has the glowlight (Voyage doesn't), which makes it easier (for me at least) to read in the dark.
Since you currently have a Kobo--you may want to get the Clara HD; unless the Voyage is just an awesome deal you can't pass it up. Switching to Amazon store/infrastructure from Kobo might be annoying (it might not; only you can answer that question).
Paperwhite (any version); don't get me wrong, I've loved my Paperwhite when it was the new thing (the original original one); but the micro improvements, even the latest flush screen (which my sister has, so I've used it for a few hours to test), doesn't compete IMO with the Clara HD for the light, nor the Voyage for text crispness.
This wasn't on your list... but the one I read the most @night is the Pocketbook Touch HD 3. It took a few weeks to get shipped (no US distributor); but I do like the lighting on it, and the text is crisp enough.