Standard practice is to use the lower front arm to adjust caster. In reality, either will adjust caster. But standard practice is to use the lower.
Adjusting the lower arm longer to add caster also slightly moves the front axle farther forward. Normally that is fine. But if you already have the axle as far forward as it can be and you still need more caster you can adjust the upper arm, making it shorter, which will also slightly move the front axle rearward. It is not so much an advantage for one or the other but a difference in how it moves the front axle. Making the lower arm longer moves the axle slightly forward, while making the lower arm shorter (reducing caster) would move the front axle rearward. The same is true for the upper arm, longer moves the axle forward and shorter moves the axle rearward. The difference is the effect on caster, longer lower is more caster and longer upper is less caster, both are moving the front axle farther forwards.
If you really like where the axle is, you might be able to adjust both in unison, making half the adjustment by making the lowert arms longer and the other half by making the upper arms shorter. That could, if the two adjustments balanced each other out, keep the axle in the same location while adding the required caster.
When adjusting this, keep an eye on your pinion angle. Both adjustments also have an effect on pinion angle, as front caster and front pinion angle are linked.