Someday I will be able to get the time to bring wireless load testing online. But until then, I can't quantify the difference.
The main reason for recommending AC1900 vs. AC1750 is AC1900 routers generally have a more powerful CPU, which can help with busy WLANs. AC1750s are also 3x3 designs, so if you're pressed for cash could be a better choice than AC1200.
For most folks - it's zero difference between AC1750 and AC1900 - the AC1750's on the market are pretty old, as not much product has been brought out - so the real steps are AC1200 and AC1900 class - and the AC1900 new product in 2016 might actually enjoy some benefit, depending on region/regulatory domain if one is in the market to upgrade/replace - the price points are pretty good there.
I think it's telling that Apple isn't doing some things - as they're pretty focused on the user experience - plug it in, it works, and it's stable...
What they're not doing...
1) MU-MIMO - they're just not going down that path, iDevices or Macs
2) AC1900 Wave 1 - meets the need
3) Mesh - they're not going there with the Airports obviously
What they are doing - Wireless direct links - if I do Airplay from my iPhone 6S or later to an AppleTV4 or Airport Express - it's point to point. and it just works - same with screen sharing from my MacBook Air 2014 (or my MacBook Pro 2012 non-retina) to the ATV4 - direct link...
Airports do bind the Guest Network to a VLAN, so when using them in AP (Bridged Mode) - it still works even on the extended AP - this is why in "extended" mode, the Guest network still works - I have not seen any other consumer Router/AP that can do this with factory firmware..
Oh Tim - BTW - the Guest Network is both bands - even on the Express - you should update the reviews there...
What Apple also brings to the table is a workable cross platform Wake on LAN/Wake on WiFI through Bonjour Sleep Proxy - and yes, this works on Windows (and Linux) with some work and SW - iTunes on Windows, and Avahi on Linux - and the benefit there is also mDNS.
What I'm getting at - the BHR vendors are running up the numbers and checklist features - and most of these seriously don't apply to most folks.
Apple isn't playing that game as it does not provide any additional value to the user... and MU is one of those things, 4 stream 11ac is another, and I would suggest that Mesh is one more that is akin to 3D TV - Mesh just makes that 3D-TV in 4K with OLED
Airports are rather simple - but they do represent what is most common and these days - the best value - not Airports specifically, see below.
The Extreme AC is basically an AC1900 class Wave 1 device - it's long in tooth in the market, but guess what - it's like the R7000 Netgear or the Asus RT-AC68U - it does the job... DLink and Linksys, along with TP-Link and others all offer product in the AC1900 class...
(my minor gripe with Apple is the current Airport Express - no excuse for having an N600 class device at that price point when it's a simple turn of silicon to AC1200 class with GbE)