A round planter is an object you place. A rectangular one is a piece of edge: set two end to end and you have a boundary, set four and you have a room divider. That is why the rectangle dominates specified work, and why the lengths matter more than the diameters. What you are choosing is how much continuous run you need and how few joints you can get away with.
Matte black suits this better than any other finish because a flat dark surface has no highlight to break the line. A gloss trough reads as a series of shiny objects along a row; a matte black one reads as one continuous edge, which is the effect most schemes are after. It also means variation in how light falls along a long run is invisible.
Length is the specification that matters here, and a single longer planter almost always looks better than two shorter ones butted together, because the joint is the thing the eye finds. Read the size table on each product page: the interior length, not the exterior, is what decides how many plants fit.
