Advice on AP upgrade

I have a big house -4500 sq ft - i currently have 2 APs one upstairs where all the bedrooms and my home office are located - that's the Asus NT66U. The other AP is one story lower in the kitchen which is centrally located and I have a cheap trendnet TEW-711BR. After Comcast upgraded my Internet to >100 Mbps I upgraded most of my network to Gigabit compatible gear. The TEW is the last piece.

Devices that connect to the APs are 2 macbooks; multiple iPhones (3 iphone6 1 iphone5) an ipad2 1...

Advice on AP upgrade