Well, IV's are much more important to consider, even if an IV is higher by 1. You may check the stat calculator here to see it for yourself.
Now, in concern to the 2 Stakataka, since the speed stat of the Pokemon is the same, a random Stakataka will be chosen to move first.
As an extra note:
If you look, at the stat calculator, Game Freak uses the floor function (the square brackets without the top), which means to round down to the nearest integer, if not already at an integer value. Because, of this, you will never see decimal values as your stat, and two Pokemon who may have 15.3 and 15.7 as their stat calculator before the floor function, will both turn out to have a stat of 15. Because of this, both Pokemon will have the same stat value, and both will be randomly chosen to move first if they were to battle each other
As for Stakataka, the difference between 0 to 1 IV may not make a difference, but two other consecutive IVs may have an effect. Try it yourself here.
Hope I helped! :)