Case 2: USB 2.0 vs. USB 3.0
I see a lot of disparaging comments about how AC adapters are crippled by using USB 2.0 vs. 3.0. The four plots below show that isn't necessarily the case.
The plots compare three AC1200 class adpaters. The NETGEAR A6200 and ASUS USB-AC53 have USB 2.0 connections and the ASUS USB-AC56 is the only one with a USB 3.0 port. It also has a 5 dBi external, upgradeable antenna.
USB 2.0 vs. 3.0
The 867 Mbps maximim wireless link rate used in the 5 GHz tests better tests USB 2.0's effective throughput of 280 Mbps than the 130 Mbps maximum link rate used in 2.4 GHz tests. But all three adapters turn in 240 - 260 Mbps maximum throughput in the 5 GHz downlink plots.
So it looks like you don't need to worry about USB 2.0 cramping your wireless style with AC1200 adapters. AC1750 USB 2.0 adapters would be a different story. But I doubt we'll ever see those.
I hope these experiments help you better understand what you do and don't get when buying a "faster" router. I've got one more set of tests to run to show what improvements (or not) moving up from "legacy" (A/B/G) and N gear might bring. Go check out the results.