From your point of view, it doesn't really matter what device you purchase. You have no problem with obtaining and converting material for your new device. Each of devices has different strengths and weaknesses.
There is one device you should look more closely at: the Astak EZ Reader
Reason is, that EZ Reader and other Jinke produced readers support hierarchical folders.
See our
wiki for all OEM dealers. Hanlin V3 from Jinke is sold under quite a few names.
Sony PRS-505 does not support folders, but it has tags, so if you use Calibre software you can organize your material quite efficiently. One book can have several tags, but tags are no hierarchical.