Basically EH > Avoidance for all bosses but the Gunship captain, but don't trade off tiny amounts of EH for huge amounts of avoidance. This is because of how bosses kill players, the majority of bosses kill players by bursting them down with in a few seconds with a good amount of them having either unavoidable damage or a stun.
In the damage potential bosses can put out with in a couple of seconds avoidance relys on RNG too much epecially if the combo that kills you has a good amount of unavoidable damage making your /praying to avoid the melee attack. An example is Gormok, he kills you with a Melee, Impale, Blled combo, only the Melee is avoidable so when the Melee hits you you will get insta-gimped if your EH isn't high enough.
Obviously there are a few exceptions, the boss where avoidance is quite useful is Gunship because the captain has a stacking debuff and when ever it hits you he gains a certain percentage increase in power, when it gets too high your group has to leave ship and slow down dps done. Also don't forget a boss like Algalon that can hit very hard and very quickly could kill you through a streak of parry gibs so in that situation expertise can be extremely helpful.