Just to throw another opinion in the mix:
Just because something is newer doesn't mean it's better. In many cases, something that's tried and tested is much better than something "newer." From what I can tell from these forums, this is especially true of Asus devices where the firmware for "newer" devices seems to need several months to iron out the issues. (It's bad enough that
@thiggins comments on it in several of his device reviews.)
Consider that router technology is several years ahead of client technology, so that 3x3 device (AC1900) is actually MORE than most current wifi clients can use. There aren't many computers with 3x3 wifi devices, and NONE with 4x4. I just shopped for a notebook computer for my wife, and I wasn't able to find a single "mainstream" notebook with a 3x3 wifi device. They're all 1x1 or 2x2. (No, you can't just upgrade from 2x2 to 3x3 in a notebook if it doesn't have the antennas for it.)
It's a similar situation with all the various technologies that are being put into the latest router generations... they only exist in the router and no one knows if those techs will ever take off in client devices.
As far as processor speed, it's easy to buy into the marketing hype, but does it matter? Processing speed isn't going to make slow wifi go faster. It's not going to make ethernet switching faster. Sure, it sounds good on paper, but unless you're using your router for something other than routing and switching, the extra processing power is going to waste. It'd be like driving a car: You can only go as fast as the car in front of you.
Of course, you asked "which one is better?" That's subjective... the 1900p is newer and has a faster (newer) processor, but the AC3200 older, has more wifi radios and the firmware is more mature. For actual wifi performance, they'll likely be so similar that you wouldn't be able to tell them apart. The same for LAN performance.
I just went from a netgear R7000 (which is one of the highest rated AC1900 routers) to a Asus AC3200 (after hitting bugs that preventing me from keeping the Asus AC3100 and AC5300.) I have about 25 clients attached to the router at any given time (about 12 wireless and the rest wired via a switch) and I found that they are all about the same. No significant difference. I don't think I ever see the CPU usage go over "barely used." (Of course, I'm using it only as a router and not as a general purpose machine.)
No matter how you end up choosing, however, I wish you luck.
Take care
Gary