I'd go for the Voyage too over the Oasis. Unless you crave the lightness and form factor of the Oasis (the display doesn't seem any better), you'd be as well going for the Voyage, or indeed the PW3 if you want to save cash.
As eschwartz has already indicated, if dictionaries are important, the Kindle is better in this respect. You'll find the Oxford Dictionary of English already pre-installed and its far more comprehensive than the Merriam-Webster dictionary that the Kobo's use.
I'm not sure on annotations & highlights other that in general the Kindle's seem to handle highlighting better than Kobo's, which is possibly because they use capacitive touch screens versus Kobo's infra-red touch.
The Kobo Aura H2O shouldn't be discounted because it was released in 2014, it has probably the nicest display and a lot of people like the slightly bigger screen on it. Where the Kobo's excel is the customisation of fonts, margins etc (though I'm not sure how this would work in relation to pdf files?). And for many people, the fact it's an epub reader while the Kindle uses it's own format.
If you're prepared to wait, you could hang around for a few months to wait and see what Kobo is up to with its potential 7.8" ereader. There's a thread about it over in the Kobo forum:
https://www.mobileread.com/forums/sho...d.php?t=275592