Bronze sits between a neutral and a color. It carries warmth without going orange, and it holds its own against brick, timber and weathered stone in a way grey does not. That makes it the usual alternative when black feels too hard for a planting scheme and white too bright for the setting.
The choice between our two is a choice about light. Metallic Bronze contains flake, so it catches sun along a curve and reads brighter at an angle than head on. It suits an entrance or anywhere the planter is meant to be noticed. Matte Bronze has no sheen at all, so it stays the same tone from every direction and recedes into planting. On a long run of identical planters, matte is the safer specification because any variation in how light falls along the row is invisible.
A point worth being exact about: these are fiberglass planters finished in bronze, not bronze planters. Solid bronze is a cast metal, priced and weighted accordingly. What this gives you is the appearance on a non-porous shell that does not corrode at the base where water sits, and a weight two people can reposition before planting.