I don't think you could say one is 'better' than the other. They are both wonderful ebook readers. It really depends on what features you like best or which suits your purpose at the moment because, really, you are not making a lifetime decision here. In a year or so you will probably be looking at something else or an additional ebook reader to supplement the one you have.
4gb is more than enough. Apparently it is equal to 3,500 books. No way most of us would have that many at once. I know I would not want to scroll through anything like that. I am sure you could move some off anyway if you were to run out of room.
I chose the Kindle because of the tie to Amazon. I have been an Amazon customer for many years and it seemed the logical choice. As well, I had heard of the Kindle previously although they weren't then available in my country. My workmate chose her Sony because she liked the touch screen and she shops for her books locally. We are both very happy with our ereaders.
So good luck. I am sure you will enjoy which ever one you finally choose
