I am a Marriott guy. First trip on the island was to the Marriott and Stellaris Casino. We had a wonderful 10 day vacation, got hooked on the island, and have been back at least once a year since. We have not stayed at the Radisson, but love The Sunset Grill restaurant and the grounds are great.
You probably not go wrong with either, but we now own at the Marriott Surf Club and really like the many options between the three Marriott.

Have Fun, Pete