To gloss, I'd add durability, and chemical resistance. The Poly Fiber manual does a pretty good job describing the tradeoffs. I sprayed a light coat of Poly-tone white under my Aero-thane orange to ensure color consistency. I used a mild paint cleaning solvent between coats, and it took the Poly-Tone off of some of the high spots. Fuel will stain it readily if you don't keep it waxed.
If you're really pinching pennies in your build, or you plan to operate your plane such that you anticipate making fabric repairs often, Poly-Tone is the obvious choice. Otherwise, the urethanes hold up better. I'm more interested in what advantages PPG offers over Aero-thane and Ranthane, which are formulated for fabric, plenty glossy, and cost less than half as much. I suppose color selection is one advantage.