Hey, Kathy, welcome to MobileRead.
Readers aren't really locked to a particular provider, though some locked to a particular file format, but those formats can generally be converted.
Several of the newer devices, such as the Cybook Gen3 you mentioned, can use multiple formats. So can the Sony, the iLiad, and the Hanlin V3.
Which is easiest to buy content for depends a bit on what, if any, file format you prefer. I think that Mobipocket is perhaps the most readily available format, which would steer you towards the Cybook or the iLiad.
For purchase content is only available in Sony's pet format from two places, but it handles RTF files and there are fairly easy to use utilities available that can convert any number of other formats to the Sony BBeB (aka LRF or LRX) format if you're willing to accept that extra work.
I believe that Bookeen (the makers of the Cybook), and I know that Sony have both committed to supporting Digital Editions .epub format whenever it finally releases for mobile devices. That alone could knock the whole availability equation on its ear.
My only other relevant thought on the Hanlin is that it's difficult to get ahold of outside of China, but that also applies (somewhat less so) to the Sony devices outside of the U.S.
Do you have an e-book format that you'd prefer to use/purchase in? That will make some difference in which device might best suit you.