Well, Kamikyokutachi is basically a best album while KnIK is an original album. If you can get the Limited Edition of KnIK then I'd say go for that.
Also if looking into getting a DVD, definitely spend the extra $$$ on AKB ga Ippai. It's expensive but it has every single PV they've made since their switch to King Records a few years ago plus the original version of Beginner and it's cheaper than getting the CD/DVD of every single so it's worth the purchase.
