Its depends on your style, ashur mist also works if you do good aggro swapping. Steel is more about solo tanking ability. As a defensive player I always make sure I have 100% critigation chance in t5t6, especially for tanking neesa which has more than 100%crit dmg, you can take like 2k less dmg if you critigate a hit. Sometime less swapping make thing more simple actually.
Dt approaching max gear: ashur mist
Guard ,conq : Steel mist ,black mist not a bad alternative but you wont get much out of it anyway.
I run a 2x ashur, 2x Steel ,2x Crawling mist on my dt since i was tired of waiting for the perfect gem.On guard i would go for crawling mist plus anything really .
All that being said i ve equiped elusive nature about 3 times in 4 years in t5 and t6 on all my tanks
As a tank class you can never get ashur up to 4 since you miss it in your amulet. And the increase from 2 up to 3 does not scale particularly good IMO. Steel does scale linear. So i personally went for 2 ashur on most of my tanks for the offensive rings and as much steel for defensive ring deck that i can reach 100% in case.