ProTip for tanking in general:
If you want to know what the best tanking set is for a given encounter (or just in general), ask the people keeping you alive. Now obviously dont ask them about details like "wats the best wai 2 gear 4 me?" or "wat def do i need?". Ask them what makes their job easier. Because chances are when you translate what makes their job easier into tank jargon, it usually produced the best results.
Typically i find that BV is much more valuable than pure avoidance (to a point obviously) due to the simple fact that low overall but very spikey incoming damage is almost always worse high overall but consistent incoming damage.
This goes twofold.
1. Consistent high incoming damage simply melds better with how people react and how this game is made (cast times on most big heals)
2. SA never having dry patches because you're a superstar that just dodged/parried/got-missed for ten seconds straight.
I would trade 1% pure avoidance at a rate of ~100-250BV where I can get my hands on it depending on my current avoidance. Less I have more valuable it becomes of course.