I think a 7-inch screen makes reading more enjoyable for me. I have 2 7-inch screen readers (Sony PRS-900 & 950) and 2 6-inch screen readers (Sony PRS-505 and Kindle 2). I always take my PRS-950 everywhere I go. But I am a girl that carries a large purse everywhere I go so for me portability is no big deal.
Plus my 7-inch Sony's are actually smaller than my 6-inch Kindle (I head Amazon fixed that with the K3).
Anyway, I think the 6-inch PRS-650 is great way to go. They are a little more expensive than the K3 because of the touch technology. That's why no one else is doing e-ink touch technology yet is because of this problem. Also the casing that surrounds the PRS-650 is not a cheap plastic so it's more studier built. I bought the K2 so I could shop at Amazon in 2009 and I will admit, I have not bought a book from Amazon since June of 2010 because I'm able to find it in ePub at the same price in many other places, that was not the case in 2008 or 2009.