The new Kindle(s) seemed to be the price leader... but if you can afford it, like SGIP says... an iPad may be a better all-around device.
The nice thing about the e-readers is lower weight, power consumption and portability... the iPad is one heavy beast. I also think the e-readers read better in sunlight than the iPad (because of the e-ink technology).
You may also wait to see what Android/Chrome offerings are coming out... and there will be PC alternatives to this whole tablet computing trend.
I did some quick web scanning and although Kindle seems to rate the highest, the Nook has some features that may be important such as expandable memory, user-replaceable battery, color touch screen interface and a wider range of downloadable media (not too sure about that part). But it also sounds like it may have more technical issues than the Kindle.
Good luck and let us know what you decide.