Progressive Muscle Relaxation is an effective intervention for relaxation, and some people find it easier to use than imagery, preferring the simple directions for clenching and then releasing various muscles. And relaxation is a wonderful thing. But all things being equal, guided imagery, which has both a relaxing component and healing content as well, is more effective than relaxation alone. It's kind of like Relaxation Plus.
In addition, many people find PMR boring and they tend not to use it after a while, whereas imagery can more easily capture and hold the imagination. The heart-opening images will also elevate serotonin levels and produce pleasurable, reinforcing feelings and reactions that motivate the end-user to keep on listening to them. In fact, in the Bennett surgery study, patients who got the imagery intervention used it an average of 2.5 times before surgery, whereas the other 3 interventions averaged only a tad over one time per subject. They just liked it better!
So all in all, I would say that for most people, imagery offers more. But some prefer the straightforward, kinesthetic directions of PMR, unencumbered by emotional content. Similarly, for some adding PMR to imagery would be an improvement; for others an annoyance. It depends on who's listening. Others find that bilateral tapping while listening to imagery renders extra benefit, and others still like to press acupoints. Often a little creative experimentation is in order, to figure out the best personal approach.
Thanks for the great question.