I know there are those who will disagree, but.... I vote for the larger pot since you are dealing with a tree. I don't like to transplant every year or two as the tree gets even larger. (I'm lazy that way.) I've never had a problem going for the biggest that is needed.

Soil choice determines what pot size is appropriate. The more water-retentive the medium is, the more critical the size of the pot. The less water retentive, the less critical the pot size. You could plant a tiny seedling in a 55 gallon drum filled with gritty mix and have no concerns the plant would be over-potted.

The depth at which roots are found in the soil column isn't a genetic trait. Roots grow wherever they find adequate moisture/ aeration/ nutrition, and they don't grow where any one of those 3 requirements is lacking. Rhododendron, in situ, are known for growing roots close to the surface; however, put them in a pot with a well-aerated medium and they will quickly colonize the entire soil mass even in very deep pots.

BTW - shallow pots are much more difficult to grow in when using a medium that supports perched water, and most commercial media support 3-6" or even more. That means if you have 4 pots of 3, 6, 9, 12" depths, and you're using a medium that supports 3" of perched water, the 3" pot would be 100% saturated after a thorough watering, the 6" would be 50% saturated, the 9" would be 33% saturated, and the 13" would be 25% saturated .

Al