Magen's Bay is absolutely gorgeous...and Trunk Bay is even nicer. Problem is, the Fantasy usually isn't in port at St Thomas long enough to make it worth the trip over. You simply do not get enough time on the beach in Trunk Bay. We did it while on a RCCL cruise where we were in port a couple of hours longer than
DCL normally stays. So, if you realistically will only get 90-120 minutes on the beach at Trunk, I would just go to Magen's Bay.