As a different way of looking to see how our green Xarm and Yarms are behaving, I looked at the locked/unlocked power on the ALS refl PDs that are used for their length locking.  In summary, the Xarm has a slightly deeper refl dip, but only by about 5%.

Looking at refl data from when the ALS arms were shuttered, I get a (very small) dark offset number for each arm.  I then look at the value of the power on each refl diode when the green arm is flashing, versus when it is locked and well aligned. 

I then look at the ratio (Locked_power + dark_offset) / (unlocked_power + dark_offset).  For Yarm this value comes out to be 0.753, while for Xarm it is 0.697, indicating that the Xarm is slightly better aligned and mode matched than the Yarm, but not by so much that I'd expect such a large difference in the ease of locking the 2 arms.