Yeah the Fire Orange is tough to photograph, but it's a rather "true" orange. The picture on the main Ultrasabers page, showing the colors, is actually very representative of their hues - it's truly a fiery orange color. Pyrestone is a deeper red-orange color, but I'll let Kham elaborate on that as I don't own that color myself.
I would say it depends really on her personality type, FO is a much more subdued orange where the PO is more "Lava" orange.
Both of the colors are sick the difference is just adding more red to one instead of the other.
I'll see if I can do a comparison for you, granted the camera lets it happen.