If you're considering the Voyage and Oasis it's probably well to think about battery longevity, the overall lifetime of the battery. A battery is typically good for between 500 and 1,000 charges. The Oasis battery is, as I recall, about 1/6 the size of the Voyage battery. That means you'll use up those charges about 6 times faster on the Oasis and have to replace the battery.
The fact that it comes with a battery and charger built into the cover shouldn't affect this calculation. The cover is simply a battery charger although hopefully a sophisticated one.
I've mentioned this a time or two and so far the only reaction I've seen is from people who say they don't care. They want it for it's form factor and if it doesn't last as long so be it. I think that's perfectly reasonable but it's not my choice. I have a Paperwhite and a Voyage. I would have bought the Oasis when it was announced except while I sat there with my finger on the mouse button they sold out. I'm glad they did because after giving it some thought I decided not to get one.
From all the reading I've done about it (I've never actually seen an Oasis) I'm pretty sure I'd like it. But I'd worry about that battery too much and that would spoil it for me.
I do enjoy the Voyage. And the Paperwhite. And my Kobo. I guess I'm not really very fussy.
Barry