Either would be a good choice. Wax myrtles tend to get wider unless you shear them some to keep them in shape. Oleander is poisonous and a bit marginal in hardiness for your area. Plant both species about 5 feet apart. One other option is the Bright N Tight Carolina Cherry Laurel. It gets much taller than 8 feet but makes a pretty good and dense hedge.
You might try contacting your County Extension Office for more hedge ideas. They have local experts in Horticulture.
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