For EInk readers, your controlling requirements are:
- Adobe ebooks
- dictionary lookup
This narrows your choices down to: Sony PRS-600 or B&N Nook, or PocketBook.
PocketBook is a bit of a wild card, the PocketBook 360° is the nicest looking 5" device on the market in the US. It supports dictionaries, but not commercial dictionaries, and has only recently been introduced in the US - it is probably best today for "early adopters".
The Nook is an unknown quantity, and is out of stock until January.
The PRS-600 has a touch screen, and the associated screen layer makes it slightly less readable than standard EInk screens. It is a full featured Reader, and therefore a bit expensive.
If you drop the dictionary requirement, there are plenty of other readers that support Adobe (PDF and ePub). See
E-book Reader Matrix.