Answer: Nurseries tend to stock the plants that sell well in a particular area. As for river birches, your choice comes down to simply a personal preference for the characteristics that a specific cultivar displays:
Betula nigra 'Duraheat'; has glossy dark green leaves; yellow fall color; aphid resistant; closely spaced leaves; bark exfoliates at early age.
Betula nigra 'Heritage'; has glossy dark green leaves; variable yellow fall color; exfoliating bark revealing cream, to salmon white color that matures to salmon brown.
Any cultivar of river birch will grow well in San Diego.
Best wishes with your landscape!
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