Marco Island
Marco Island has a wonderful beach and clear water. The Marriott hotel is just a short walk down the beach from the timeshares and is one of Marriott's best resort hotels.
As far as the city of Marco Island is concerned: it's a small island and is mostly residential with smaller (but expensive) houses and condos. Do not expect to see "a strip" with every national restaurant and retail chain.
The difference between Macro Island and Myrtle Beach is like night and day. They are very different and I'm sure many will have strong opinions, but it simply comes down to the kind of experience and atmosphere you're looking for.
The Marriott timeshare (Ocean Watch) at Myrtle Beach is a fantastic resort (yes we own there, however we are not "Myrtle Beach" people, but the resort itself is tops). Myrtle Beach is a popular destination and is undergoing a change. Some have called it the "redneck riveria." That label comes because for generations Myrtle Beach was the beach for "regular working-class" people. It featured more t-shirt shops, pancake houses and miniature golf courses than anyone could count. Hilton Head was the more exclusive and upscale South Carolina beach.
But Myrtle Beach has passed a number of laws to encourage better land planning and zoning and is transforming itself. There are parts of Myrtle Beach that are very, very nice.
As far as the beaches are concerned, Marco has clearer water and softer/whiter sand. Myrtle Beach and Hilton Head have "east coast gray" water and coarser sand. But MB and HH have a lot more to do off of the resort property (fishing, boating, shopping, golf, kid activities, shows, etc).
Marco would be more "relaxing."