CNET recently revisited this question. No real surprises in the article:
Kindle and Nook for the sub $150 and Kindle 3G and Nook 3G for the sub $200 and in the $350+ range the iPad and Kindle DX
http://news.cnet.com/8301-17938_105-20009738-1.html
Actually the Nook touchscreen colour $250 may well be the joker in the pack as it is poised halfway between the Kindle 3 and the iPad and you thus get Kindle + lots of things for just 100 USD more or not much less than an iPad at 250 USD less.