Block rating and value are... overrated. For any serious boss fights. If you're interested in survivability, go with the dodge trinket every time. Not only is the "use" effect immensely better, but the "equip" stats aren't even close to equal. Which is better: a 5% chance to mitigate 1500-3000 (depending on shield block being up and trinket popped) damage from a 20k attack? Or a 2% chance to totally avoid that same 20k attack.
Shield block rating won't save you on boss fights where you're worried about surviving the encounter. Neither will an extra 400 block value. That's a threat trinket or a trash trinket, where you have many mobs attacking for lower amounts.
When it comes to burst survivability for bosses, the base dodge on the second trinket combined with the extra "use" dodge boost pwns.
"Stop trying to hit me and hit me!"