Answer: The plants in question are very similar; both are members of the same genus, just different species. Thuja occidentalis is the American arborvitae; Thuja plicata is the western red cedar. The Giant Western Arborvitae is classified as Thuja plicata; the Green Giant Cypress is a hybrid between Thuja plicata and Thuja standishii (a Japanese species). It is less cold-tolerant than Thuja occidentalis, but that shouldn't be a problem in your mild winter gardening region. Hope this information clarifies rather than confuses things!
Green Giant can grow 3-5' a year, ultimately reaching 30-50' tall and 10-20' wide. If you're looking for the fastest growing plant, Green Giant is the right choice.
Best wishes with your landscape!
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