Hilton Head Island is more upscale, quiet, and classy. There are lots of good restaurants, bike paths, golf, tennis, and a beautiful beach. The island is heavily forested with a wonderful natural landscape despite being fairly heavily populated. There are no neon signs, and strict zoning maintains the refined ambiance. Beaufort and Savannah, both nearby, are filled with antebellum architecture, history, and charm.
In contrast, Myrtle beach is crowded, noisy in areas, and somewhat honky tonk. Highrise condos and hotels line the beach for miles. There is a lot of shopping, live music shows, and probably more miniature golf courses and waterslides than anywhere in America. There are also a huge number of regular golf courses, a fair number of motorcyles, and on the north side of town, some upscale areas. Myrtle Beach is much more of a traditional beach town that has simply mushroomed after decades of rapid growth.
Steve