Kobo has more choices for configuring the layout of an ebook -- font, size, line height, margin overrides, etc. even without resorting to patches. Some people find this helps them focus better on the book.
Personally, I am fine with the limited Kindle defaults.
Of course, I luckily have an older model Kindle that was able to be jailbroken* and I can therefore use the various hacks that have been designed for the Kindle.
I mainly use the screensavers hack, the Collections Manager hack, and the alternative browser.
* -- without opening the case to get at the serial port.