I love my Nook and the 2 things that I think made Nook superior than Sony (my opinion only of course) are:
1. The feel/look of the device - I like the sleeker look and the feel of Nook compared to Sony.
2. B&N bookstore vs. Sony - I can go to B&N brick and mortar store to get help if I need to.
Other than that I think both device has the same capability.
Sony ereader has touch screen while Nook only has touchscreen on the bottom part of the device. Some say that makes sony screen not great for reading (glare issue).
Nook however is a bit heavier (about 1.5oz heavier I think) than Sony 600
And nook is a bit thicker as well 0.1" thicker than Sony.
Sony is also a bit more friendly with pdf format. Supposedly you can read documents both landscape and portrait. But I don't have Sony so I can't say.
Some people don't like Nook's touchscreen control. I personally think it's awesome. Then again I'm not impatient to get stuff going. I can't say if Sony's touchscreen control is any better or worse.
If your purpose is strictly overdrive book reading, either one would work.
It'll depend solely on your personal preferrence of the looks, feel and reading experience on each device.
Go to
Best Buy and see both devices... maybe one will capture your heart more than the other once you see it in person.
Good luck!
Oh and there's been rumors that next gen Nook is coming soon, but nobody could confirm whether soon means this fall or next January or who knows when. If it is this fall, you might get lucky and find first gen Nook out cheaper soon.