I don't own either, but I did find some facts that might help you make an informed decision.
Each vehicle is rated to tow up to 8,500 pounds. The Armada rides on P275/60R20 tires, versus P275/50R22 rubber on the QX80.
Armada - 390 hp and 394 pound-feet of torque
QX80 - 400 hp and 413 lb-feet of torque
There is a little bit more legroom in the back on the 2017 QX80 than in the 2017 Armada; you’ll get 41 inches in the Infiniti and only 39 in the Nissan. The front seats, however, are the opposite; The Armada has 41.9 inches of legroom, while the QX80 has 39.6 inches. Both have cargo volume of 16.6 cubic feet.
The Armada utilizes electric power steering, while the QX80 is hydraulic. The Armada runs on regular unleaded fuel, whereas the QX80 requires more expensive premium fuel.
With the more expensive QX80's Limited Trim for example, you get extra features such a power tilt-down mirrors, rear spoiler, roof rack, running boards and LED high beam/fog lights. The interiors of the two vehicles share almost all of the same features, although the QX80 adds rear seat audio controls, 2 more speakers, and a fold-flat front passenger seat.