So what is the question? Which works best for you? Which is easier to use? Which has best support? ... Be specific on your question.
Instead of asking why not share your experience, both are well known for pop under and paying fast, I don't think others review is going to help you to decide which one will work good for you, as both has their own market and you will mix review, so just go whichever work good for you.
They both works great if you are a publisher, but I've never tried being a advertiser. PopCash gives me a higher CPM, but PopAds gives me a higher quantity (I have no limit for the pop-ups on a few of my sites).
i dont recommand popcash, they have A LOT of bad advertising. Fake Flash. Android Locker. Scam Support => http://malvertising.stopmalwares.com/2015/03/plugrush-prpops-com-and-popcash-networks-malvertising/
Popads are the best in the popup / popunder business, period. Check this ad network review site for more information and ratings-> Popupreviews.com