I like the 91% one. I think Stamina is overvalued, but not so much that 15A 13D is worth 3 points of it over 13A 15D. I also think break points are overvalued vs. bulk points (which is better, one point more on 5-9 quick attacks, or one extra charged attack?) so I have no problem with 13A 15D.
But, if your resources are limited, there's plenty of other good counters to Regirock, including Machamp (who is good for all three), and SB Groudon. Kyogre is a beast, but as you say, Regirock is it's only likely use in the next six months or so for legendary raids. And going forward, you might want all three above powered up, but that's a problem for another month.