nVidia makes the chip and a basic fan for the chip. Other companies, like those you listed, take this model and make it better by adding a better fan, OC'ing it and some other small stuff.
Yeah, buying from different companies is mostly subjective but it can also be a matter of like, is their customer supprt response good? Do they respect their warranties? Do a lot of their cards come DOA? (dead on arrival) Other than the company's reputation, its really just preference. Oh, some companies have different fans on their cards so they might need a different PSU or less/more coolers.