I have all 3 routers, the WRT1900AC v1, RT-AC68P, and R7000. Of these the WRT1900AC can perform the best, but there's currently not any firmware out there generally available that makes me happy, so I don't use it much. I'm also very unhappy with the rumors that Linksys has dropped support for the WRT1900AC v1 already, which steers me away from Linksys bigtime. And the price differential, Linksys has apparently gone to luxury pricing, isn't worth the difference in performance (or having to put up with firmware that is lame for a higher price) to me.
I picked the R7000 over 1.5 years ago, and while the firmware has been a bumpy road, you now have stock, tomato ARM (that's what I'm using), dd-wrt, and RMerlin on the R7000 all available and mature. All good choices with different strong points.
I have found the RT-AC68P not to stand out in any category at my place, other than RMerlin's firmware...the wireless is not as good as either the WRT1900AC or R7000. So the router that I usually use is the R7000...great performance. And the IPv6 works well with stock, dd-wrt, and tomato ARM firmware, not with RMerlin on the R7000 for me, but it works for others. My IPv6 here tends to be a pathological case, difficult for firmware developers to get working correctly. IPv6 has been working on the stock firmware for some months...I haven't tested the latest stock firmware, but I'm hearing that they've fixed their icmpv6 packet filtering problem as well.
Anyways, I'd strongly recommend the R7000 of these 3 routers from my experience.