Thoughts from someone who has neither but is tempted by both:
1. Agree with others that being able to load PDF and read them as well as DRMed books is essential.
2. Problem with PDFs is that it is a fixed size. The screens on these readers are small so looking at a PDF formatted on a letter size page is going to be difficult.
3. Paperback size is overrated but slimness is essential. I'm thinking maybe a 7"x10". This sort of overcomes the PDF problem so that there is no need to translate a PDF to a native Kindle or Sony Reader format.
4. For the $300-$400 price range I'd like something more than just a reader. The reason I don't carry my laptop around is the bootup time. I'd like something that has a dual bootup option - full operating system or a strip down version that will just quickly load the reader or even play music (e.g. MP3 or an Ipod like ability). Ability to just use a reader or to play tunes would also not suck as much battery juice.
5. So yes, I'd be willing to pay more for a small laptop that does all of the above perhaps $800-$1000.
6. Unfortunately, a laptop like the one I imagine might be heavy. Too bad.
7. I'm predicting an eventual joint venture between Apple and either Kindle/Amazon or Sony to produce something like that. (I wish!)
8. I like Kindle's bundling of Wifi and download without any additional charges. Of course, its been bundled into the price of the Kindle books.
9. Which brings me to the complaint that $9.99 per Kindle book (on average) is way too much. I'd go for something like $4.99.