Aftering doign 10 man ICC and ToGC 25 I've found this.
You're current Highest Stat is chosen.
When Heroism and my Engy Hast eGloves are Procced I -always- get a Vrykul.
Otherwise It's random between the Taunka (Str) and Iron Dwarf (Crit)
But those are the only 3 I ever get.
I traded the trinket to a hunter while it had 2 hours left on it to try it:
So I'm assuming it picks 3 for every class/spec.