As much as I would like to say STR, the devastating impacts of stun on the Measure system heavily hints at STA, if anything for the annoyance of having to restart a full measure from scratch when you get stunned.
I can see AGI being more impactful, but I think (not sure) it's tougher to gear for AGI.
I would probably go all out on one or the other, dependening on the race, but I'm sure there is an optimal answer to be found.