My OPINION is that any eInk device is going to be better than any backlit (tablet/phone) device for long reading sessions. If you do a lot of graphic novels, manga, or magazines, they're probably better on a tablet.
I do find that the light really helps with contrast so I'd look at the Paperwhite as a minimum. The regular kindle is good, but the lack of built in light limits its utility. Paperwhite is probably most bang-for-the-buck so that's the way to jump if finances are a consideration. The Voyage is the lightest of the bunch but that's really the only advantage over the Paperwhite at this point. The Oasis is obviously their top of the line product. Screen is bigger, lighting is great, buttons simplify page turns. And it's more configurable in terms of accessibility.
As noted, Amazon has a pretty generous return policy . . . in fact, during the 'Holiday' return period which runs through the end of December, you have until the end of January to return for refund. They may charge a return shipping fee.
If most of your books are ePub, you might be better off looking at the Kobo . . . . there are several models with comparable features. That would mean you don't need to do the ePub conversion. And I believe it can utilize .mobi files as well.