Head to head
Which one will break you first?
Two fingerprints on one grid. Both are scored through your profile, so this answers the question for you rather than for the average player.
Age of Empires II: Definitive Edition is the harder of the two for you, by 1.8 points. The gap is widest on learning curve, where Age of Empires II: Definitive Edition asks considerably more than Metal Gear Solid V: The Phantom Pain. If you want the gentler run, start with Metal Gear Solid V: The Phantom Pain.
Axis by axis, biggest gap first
METAL GEAR SOLID V: THE PHANTOM PAIN
Reflex mode quietly saves every blown infiltration. Turn it off and the enemy adapts to whatever you did last mission, which is where the game gets interesting.
You will lose sometimes. You will not mind.
AGE OF EMPIRES II: DEFINITIVE EDITION
Build orders are the curve, and the campaigns teach almost none of what the game actually asks for. The hard AI does not forget its economy while it fights.
Sustained pressure. Progress is earned.