Simple answer: Both engines are exactly the same thing. Yes, maybe the computer on the QX80 has been tweaked to make it produce 400hp when the car is using PREMIUM only.
Anybody here who drives a QX and pays attention to how their car handles will agree with me that when you consistently use PREMIUM gas, your QX performs great! It is responsive and gets up and go. If on the same car, you have used regular fuel for a long time also, you will know that regular gas makes the car a little less responsive and sluggish as compared to when it uses PREMIUM all the time. Even Nissan says PREMIUM IS RECOMMENDED FOR MAXIMUM PERFORMANCE. And that is very true on the QX.
Stock 2018 QX which faithfuly uses premium has 400hp. If this car uses only regular, I can guarantee it gets only 390hp.
Stock 2018 Armada which uses regular has 390hp.
You can see a stock 2018 Armada compared to those QX owners who only use regular probably both have only 390hp. This is not scientific, but I do believe it as a QX who consistently use regular is slightly sluggish compared to a QX which consistently uses premium.
has the kn airfilter that claims another increase of 10hp. My guess is that a 2017/2018 Armada can use this same filter and may probably see a boost of 10hp which will move its hp from 390hp to 400hp and thereby matching the 400hp of the stock QX80.
At this stage, if you put the most loaded 2018 QX80 vs the most loaded 2018 Armada, the Armada is truly a great buy. The only thing the QX owner can be proud of is the HBMC option, which is not an engine component but suspension. And the price difference for both most loaded options makes the Armada a truly great buy.
To conclude, both engines are the same.