You'll never read comfortable on an iPad outdoor on a sunny day. So if you ever want to do this your best choice would be eink.
If you sometimes read outside and there is not always direct sunlight you'll have to make a decision of priorities and i just can advice you to test it on your own before making a decision.
If you are the inside reader type its much easier. You'll then just have to find out 1st if you really need all the extra gadget stuff an iPad offers like Apps, Video, Webbrowsing, etc. cause an iPad goes for 499$ and a K3 for 135$ so if you just need a reading device get a K3 and save your money.
The 2nd decision you'll have to make is if you can stand the shiny display of the iPad. To find out you can sit in front of your PC, if your eyes don't strain after a few hours why should they when using the iPad? But if they do so you should try out eink, much less pain for your eyes if you have problems with reflective screens.
And never forget the only advantage of an iPad when it comes to reading experience is that you will have a better reading experience in a dark place. But you can always fix that with an 9.99$ light for your eink screen.
So the most important question for me would be do i need a reading device our a multimedia device with reading capabilities
