Both are nice beaches, just different. Orient Beach is very commercial and you might have nude sightings there since there is a nude beach way down the end. Just FYI in case that is offensive to you. Me I personally prefer Dawn Beach which is not quite as commercial as Orient Beach and has a great beachside restaurant and bar. But no matter which you choose, you should have an enjoyable day.
Just as another FYI, you can do St Maarten on your own quite easily. We usually rent a car if we are gonna be doing several different things or a taxi works great too.