I've used both. I'd go with smashbox. I dont like De slick in a tube for primer. To mattify over makeup after a hot day in the kitchen maybe but not as a primer. Actually UD doesn't advertise De-slick as a primer.
the ingredients are:
Aqua (Water), Magnesium Aluminum Silicate, Propylene Glycol, Allantoin, Phenoxyethanol, Caprylyl Glycol, Ethylhexylglycerin, Hexylene Glycol, Fragrance
as opposed to smashbox's photo finish primer ingredients;
Cyclopentasiloxane, Dimethicone Crosspolymer, Dimethicone, Trisiloxane, Silica, Dimethicone/ Vinyldimethicone Crosspolymer, Ethylhexyl Salicylate, Retinyl Palmitate, Tocopheryl Acetate, Carthamus Tinctorius (Safflower) Seed Extract, Propylene Glycol, Water (Aqua, Eau), Vitis Vinifera (Grape) Seed Extract, Cola Acuminata Seed Extract, Camellia Oleifera Leaf Extract.
the "..cone" endings are indicative of primers and De-slick has none.