I only have the 300, but I've played with the 600 a little in the store. I think the 600 is faster for page turns, but the 300 is very reasonable to me. The matrix at
https://wiki.mobileread.com/wiki/E-book_Reader_Matrix shows that the 600 has a much faster processor, which makes sense: The 300, while a great reading device, is Sony's low-end device and has a smaller screen, slower CPU, no built-in dictionary, etc. In other words, the 300 probably won't ever match the speed of the 600.
My understanding of the battery issue is that you're going to be using it up every time you turn a page. It's likely (I'm not an expert), that the amount of text on the page has very little to do with the amount of power it takes to refresh the screen. But again, I'm only guessing.
With both of those in mind, I still think the 300 is a great reading device. It's smaller size is a plus if you're on the go, and the battery life will last you several books between charges.