It all really depends on what batch size you are trying to accomplish. The larger square coolers can be had for larger volumes for cheap, but the round ones offer a smaller dead volume. Also your budget matters. Also how big of a beer do you want to make In terms of OG?
Those two. Stick with a round MLT for ease of build and use, unless you need the much larger grain capacity offered by chest coolers for either larger batch sizes or for big beers...