Hi,
We have both the Sony Reader (1st gen prs500) and Kindle.
If dictionary lookups are most important, Kindle wins.
You can lookup in context to what you are reading and also Wikipedia and googl e it with the wireless.
The non-wireless readers are limited by the dictionary corpus.
I love the fact that the search function works across all indexed books and documents. Unfortunately, Kindle doesn't have landscape nor zoom like the Sony. I can make out my pdfs better with the Sony than with Kindle or MobiPocket Creator conversions.
That said, however,
I love my Kindle