The Tadlow is an excellent re-recording -- except for the tonic ending to "Pax Romana". That version would be okay for a concert performance of the piece in isolation, but it's nonsensical s as part of the dramatic flow in the sequential film score.
I have a recording of an old extended interview with Tiomkin. During it an abbreviated version of the "Pax Romana" is played (obviously from the music sessions) - which ends the same way as the Prometheus recording. I suspect that's the way it was originally composed, and either for the session the original ending was truncated and the cue was instead concluded with a sustained chord; or Tiomkin composed an alternate ending that was also recorded. I don't believe Prometheus would simply compose a new ending for the cue.
I'm always perplexed by this type of topic. Why must one choose between one or the other? If you really like the music you'll find a way to buy both versions.
Frank K. DeWald's notes on the Prometheus release clearly state that the ending of "Pax Romana" uses Tiomkin's concert arrangement.
Personally I wouldn't want to be without both releases.
I have both versions. In the film, the delegates from all over the empire gather to the processional, "Pax Romana", and they are all wondering, what is this all about ? What is the emperor going to say? So the piece ends on that famous note of expectancy. The film version supports the unfolding drama, the concert version does not. As you say, both are good, but I believe a sequential representation of the score should go for the film version.
