Get the older (X6) tri-band or newer (X4S) mu-mimo/faster CPU/Dual-USB3 etc etc at good price.
Personally, just got the X6 as our home devices (phones, laptops etc) not the latest so not benefit from mu-mimo (also have dedicated media server so no need for USB ports - and will be up on the wall so won't be able to reach - X6 has one USB3 if needed). Newer X4S would be MUCH better option if you have newer devices (also much faster CPU if you want it for 'heavy' VPN usage etc).
X4S clearly more 'future proof' but by the time our home kit is replaced (2-3 years) something far better will probably be out. Also don't intend flashing (Tomato/DD-WRT) and most X6 bugs ironed out by now so will be sticking with official upgrades.