I can only speak about reading cbz/cbr comics....
I have found that most 7" tablets are not quite good enough for reading comics. The only exception has been the 7" Kindle Fire HD 16GB. It is actually very readable. I just tried reading a cbz-version of an old 1971 Sears Wishbook catalog. The original size was at least 8.5" x 11.0" and I was pleasantly surprised to be able to read the catalog without the continuous need to zoom in.
If you can, try to get a hands-on with both 7" and 8.9" models. The 7" MIGHT be acceptable to you, but I think that there'll be absolutely no problem with the 8.9".