When it comes to the cheap oiled air compressors that are found at stores like Harbor Freight, Northern Tools, Homier, etc., there are really only two manufacturers, Wen and CTG. These two Chinese companies actually manufacture the vast majority of these compressors, which are then rebranded and sold through a number of channels. I don't have enough experience to comment on the relative quality of their products, but I will discuss their differences and how to recognize each one.
Wen manufactures a variety of air tools and pancake, "hot dog", twin tank, and small horizontal compressors. Their compressors, except the 8 gallon horizontal, use the trademark Wen pump unit. This pump unit is usually advertised from 1.5-2 "HP" and most commonly spins at 3450 rpm and produces 4.2 CFM at 90 psi. Wen also makes a 1720 rpm pump unit found on the HF 94734 that produces 3.8 CFM at 90 psi and is significantly quieter. Wen sells directly to the public through their website, and also through many retailers. The easiest way to recognize a Wen compressor is to look at the rear of pump unit: