If the ducts are used for cooling as well as heating, if you went with air-permeable insulation you'd run into condensation issues and possible corrosion of metal ductwork unless you keep the basement at a VERY low humidity. Foil facers (even those with perforations) limit air-exchange between the insulation and humid room air, limiting the amount of condensation forming on ducts.
Before you insulate it, be sure to seal every seam & joint with duct-mastic. Half the noise from metal ducts is the hissing & whistling of leak points.
There's no ignition-barrier requirement for fiberglass insulation or it's facers on HVAC ductwork, as long as it meets minimum clearances from flues.
A flash-foam of 1" closed cell goods (FrothPak, TigerFoam etc) would air-seal and insulate to R6, but WOULD need an ignition barrier between the foam and conditioned space. Closing in the soffit with half-inch gypsum would meet code. It's a pretty expensive way to go if you have a lot of duct to insulate, but it's air-impermeable and a good sealant on it's own.