An eBook reader is a single purpose device - all you can do (with minor exceptions) is read books. A netbook is a general purpose device - you can surf the web, read books, compose your next novel, etc.
I have a Kindle so I can give some feedback on battery life there. If you keep the wireless turned off except for when you actually need it (downloading new books), you can get a week or more of battery life before you need to plug it in.
I last charged a week ago, then hopped on a 6 hour flight and read the whole time. I've used it sporadically since I've been back home (I've been reading dead tree books instead mostly). It still has about 80% charge. With the wireless off, the only time it really uses electricity is when you change a page.