I dropped Yotei at 12 hours and got bored of AC around 30, so I voted for AC.  AC is still fun in small chunks every now and then, so maybe one day I will finish it.  I wouldn't say either are great games.  AC has the edge for me due to the stealth gameplay being fun, better parkour and I prefer the populated, more realistic looking world.  GoY has really bad stealth, parkour and the world doesn't feel alive at all.  GoY has better melee fighting, but even that got boring pretty quick.  AC melee fights are really clunky and bad.