x2600 and x3200 indicate the vertical resolution of the images, so the latter is obviously bigger than the former in that regard. In addition the x3200 version has color images saved as PNGs because people requested for it, while the x2600 uses high-quality JPG compression to keep down the size (but it doesn't really affect the perceived quality unless you zoom really close in).
Here's what you should consider when choosing what to download: What is the resolution of your biggest monitor that you use to read the books, and at what size will the images be displayed? If the vertical display resolution of the images is smaller than 2600 pixels, then you should most definitely grab the x2600 version if you also want to keep your library size lean. And if you upgrade your setup in the future, you can always come back to download the x3200 version later.