If you just plan on reading books, go for the 8GB. If you have any interest in audio books, go for the larger one. Comics can get up there in size too, but depending on how many you want to have on there at one time, you're likely fine with either.
Yes, I recommend using calibre e-book management for this purpose. Web2Epub is also useful if you wish to convert web serials to epub for later reading on your kindle.