If those are your two options, go south from each destination. For VA Beach, try Sandbridge. For Myrtle Beach, try Litchfield or Pawley's Island. These are quieter, more family oriented areas. Even nicer are Kiawah Island or even Hilton Head. I prefer MB over VB. However, I am a beach snob and have been fortunate enough to be spoiled for the last 10 years with frequent visits to amazing beaches far from the east coast.
I don't like Va Beach at all (actually heading there this week for a softball tournament) and I definitely would not want to stay near the main, central section. There are a lot of activities along the main section - good for a night, but I wouldn't want to be near here all the time. Beach itself was gross crowded and dirty- more so than anything I've seen at MB. We did Myrtle beach a lot when I was growing up and it's busy, with lots of beach, lots of people, lots of restaurants, ziplining, mini golf, and regular golf. We've been there a few times in recent years (last November for softball). Again, like VB I wouldn't stay in the main, regular incorporated MB with only one or two exceptions, but those are pricey (Marriott resort).
For either location, use TripAdvisor and look at the pictures people post. There are tons of hotels and rental units.
YMMV.