Agree with RMerlin.
A few further thoughts. With the RT-AC68U 'original' being almost 3 years old, the latest iterations will (hopefully) have all that Asus has learned over the last few years, in addition to the latest and most capable hardware.
http://www.techhive.com/article/208...-router-on-the-market-priced-accordingly.html
$40 extra to 'upgrade' to 3 years worth of progress is an easy decision (particularly as it is still less than when it originally launched). The icing on the cake is that the RMerlin firmware is directly usable on this new hardware with no workarounds needed.
Depending on your network requirements and use today, the RT-AC1900P may be 'more' router than you need.
But, that will not be true when old father time takes it's toll over the next few years.
Not only will you and your devices/clients increasingly demand more of the router you buy today, but the firmware itself will too as it is constantly beefed up with security and usability features that may get you to re-evaluate what you expect your router to do for you.
Something I'm curious about, what router do you have now?