If you're in one of those eligible groups, I've run across some preliminary statistics reports showing that Moderna is slightly more effective against the Delta variant than Pfizer. This from looking at people who have had two doses, not three, but If I had to start taking massive doses of corticosteroids (putting me into an eligible group), and both were available, I'd pick Moderna.

Again, preliminary data, but possibly backed up by the larger number of post-second-dose reactions with Moderna, indicating a greater immune response.