I don't think you'd have to worry about the battery needing to be changed by Sony for many, many years. You can charge your reader on the USB on your computer (not while it's in sleep mode) or buy a PSP charger with the barrel plug to charge it in the wall. You'll be good to go for a long time. Heck, you might even buy a new reader before you have to worry about getting a new battery from Sony.
I have a 300 but since we're comparing the 300 to the 600, one of the things I want to know about is pictures in books. If I buy a book that has pictures in it, will I get to see those in my reader? Does it depend on the device I'm using or does it depend on the publisher including them in the ebook version or both? Just curious to know.