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.
Rainbow Six Siege X is the harder of the two for you, by 1.4 points. The gap is widest on reaction, where Rainbow Six Siege X asks considerably more than Age of Empires II: Definitive Edition. If you want the gentler run, start with Age of Empires II: Definitive Edition.
Axis by axis, biggest gap first
RAINBOW SIX SIEGE X
Every wall, floor and ceiling is destructible and every one of seventy-odd operators changes what a room means, so the hardest part is a memory problem rather than an aiming one.
The game is the antagonist. Good luck.
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.