I agree that the display size is definitely worth it, IMHO. Also, it is possible to increase the flash storage of the Kindle DX. If you are willing to put the time and effort and you have a big enough need, it can be done. I am currently doing it for the Kindle 2 and I have not run into any problems.
See one one my previous threads here
https://www.mobileread.com/forums/showthread.php?t=92772