I also had WRT1900AC version 1, comparing with WRT1900ACS. You can't feel the difference between two although WRT1900ACS with faster CPU( from 1.2GHz to 1.6GHz) and more ram: 512MB, it's interesting WRT1900ACS canceled cooling fan, why
light-point on hardware, very powerful: dual-core 1.6GHz CPU, 128MB flash; 512MB RAM, high capacity to link many devices, good cell apps, USB3.0 port, no problem to stream HD video, could be a family NAS, openwrt, dd-wrt compatible
poor factory web-gui; no QOS; wifi range is not impressive; no real VPN; pricey; poor package ( refurbished product