Totally different vibes to each. We enjoy both for different reasons.
Since you are not really familiar with either-- you're probably going to end up using a car at either place.
These are not places where you park your car and walk to restaurants, shopping centers and hop on a water taxi.
Generally speaking-- The Myrtle Beach area itself is hotel beside hotel beside hotel. Historically, they were mom and pop run hotels that grew into large complexes with additions over the years. Because these mom and pop hotel operators got the land back in the 50's, there historically wasn't real estate for big hotel developers to come in. That explains why there are no large luxury big hotel name resorts... the good land is simply not available. The large chains are waiting for these old mom and pop operators to age out and sell. Marriott told me it took them more than 10-years to buy the land where the Ocean Watch timeshare and the hotel are now located. I wouldn't call MB a "pretty" place-- it's a jumbled hodgepodge-- But MB is popular. The Marriott is bit away from the "heart" of Myrtle Beach-- which gives it a different feel from the rest of MB. It's in an upscale development (Grande Dunes)-- expensive houses, condos and a country club beach house are the immediate neighbors. So there is a "little" bit of space from the "hotel beside hotel beside hotel" congestion. Many go to Marriott Ocean Watch and do not leave the complex. It's a bubble from the rest of Myrtle Beach. Not close enough to walk, but a quick drive is every type of restaurant you could imagine. This is a tourist town and the restaurants are typically large and hold a lot of people. There are a nice shopping areas, excellent live shows and movies. But there are also PLENTY of t-shirt shops. MB caters to every taste. You will see it all in MB.
Hilton Head is visually "natural" with shades of green and brown. Zoning laws require trees and bushes that basically hide shopping centers and buildings. You can't just cruise down the street and pick a restaurant because you can't "see" the restaurant. You almost have to know the name and address of where you are going when you leave your hotel. It's not urban at all. It's the opposite of MB-- you will not see a strip of hotel beside hotel beside hotel. It's known for wide meandering bike paths that crisscross the "island." The vibe is more relaxing. Things are spread out -- plenty of walking pathways-- for exercise. But you will have to get in a car to go shopping, to restaurants or to take part in an activity (canoe tours for example). There are outlets and small boutique shops.
With that said, at both places there are plenty of people who arrive, go to the grocery store to stock up for the week, and never get in their car again until the end of their stay. On most vacations we are usually "doers"-- but at both MB and HHI we stay on property most of the time. We walk the beach, walk the pathways, play tennis, ride bikes and go to the pools. We don't plop in a chair and sit for a week, but we do stay mostly on property. We venture out for dinner once or twice but that's about it.
I hope this helps give you a better visual image.
