If I was being paid to play, I would use mediums (John Pearse PB). In terms of sound, mediums are going to bring out the absolute best in a well-built acoustic.
I think most full-size Guild flattops offered thru the years have come with mediums from the factory and can withstand the tension.
But, especially on the less heavily built models with scalloped bracing, they may not tolerate that tension all the time and forever. My F-44 developed too much belly years ago, so I backed off to lights.
Liight strings are easier on the player and on the instrument. And I'm more likely to get paid not to play. So I use Pearse PB lights (12-53). Compared to some light acoustic string sets, these are closer to a medium set than many others.
Sound appreciation is subjective, though, and I can certainly understand someone believing that lights sound best to them on a particular guitar.