A little reminder: older versions are NOT recommended due to various security fixes over time.
And I will not be so sure to go for the RT-AC87U right away...
To have benefit of the extreme features (4 spatial streams and much more), your clients must support this as well.
Also realise that developments of 802.11ac will continue for a while.
The AC87U is only halfway of what the 802.11ac standard offers (8 spatial streams, 160 MHz bandwidth, 256 QAM), again...not only the routers shall get enhanced, the clients as have to keep pace as well.
Asus just very recently introduced DFS for the RT-AC68U with version 3.0.0.4.374.4561, so it is all still quite experimental.
Also the RT-AC87U will go through a firmware learning curve.