I feel a fixed retainer with an Essix retainer helps in combating the high tendency of relapse associated with maxillary diastema. Additionally, if the fixed retainer is debonded, the essix retainer can serve as a primary retainer till the patient can see the orthodontist for new bonded retainers.
a combination of both thats for sure and I prefer a HR or something that has a labial bow , we can never guarantee that the fixed retainer will stick . if the diastema opens again the retainer with the labial bow can be tightened to help close the gap. on the other hand I sometimes add some flowable composite incisally ( not too much) between the 2 centrals lingually just as an addition to maintain the space closed