Sigils of strengt
Fire has more upfront burst.
Like Aro said, Sigil of Fire will be more front-loaded and useful against trash mobs and short battles. Sigil of Strength will only be more useful for longer fights (bosses/15+ seconds) or for areas where you have continuous targets (farm areas/temples/etc) BUT ONLY IF you have a high Crit Chance AND good +Boon Duration (fast attack speed is preferred). You can easily achieve 10 or so stacks under these conditions. For any play outside of these conditions, you’re better off going with Fire. In the right build Strength can be VERY effective though as it scales your damage dramatically if you don’t have easy access to might otherwise.
will both of them proc or will only one? I realize two of the same is one wasted, but will two crit-proc sigils like fire and air both proc fairly equally? And which one is checked first, main-hand?
They’re all on separate ICDs now. Air will proc more often though because it’s CD is 3 secs vs. Fire’s 5 secs. Only ‘stacking’ sigils can’t be used together.