September sounds like a great time to do both (and is when I used to do all my trips, usually great weather everywhere and no summer rush of families as kids are in school), do you have to pick just one?
We went a few years ago in January which was fantastic because there was no one around, had the hole place to ourselves, didn't suffer through stifling summer heat, and Bryce, while always amazing looking, was stunning all dressed in snow, and we never needed more than a sweater. But, unfortunately Zion was not as good in the winter, couldn't do the narrows or Angel's Landing because of ice. We also did the Grand canyon then which was also excellent in the winter.