My wife and I have the SONY PRS-600 and the B&N nook (original with 3G) in the house. We mostly use them for library books to read in bed - and so for us that's the key to our strong preference. The nook beats the PRS-600 "hands down" in that the SONY has a firmware defect that prevents you from using BOTH epub and pdf library books at the same time. They have a support link that basically tells you to avoid trying (see below)
I also think the nook's screen is more "paper white" than the SONY and therefore incrementally better contrast, especially in lower light situations.
The nook has since dropped price a few times and recently B&N has introduced a new model without 3G feature. Unless you need to buy books on the road (via AT&T 3G) when you are nowhere near a wifi connection (which all nooks still include anyhow), then the current non-3G nook would be my first choice.
In fact, if we could do it over again, we'd return both of our current readers for a pair of (non-3G) nooks.
Happy hunting.
SONY link:
http://www.kb.sony.com/selfservice/d...nalId=C1008686