think 6x6 has advantages vs 6x4.5 if you like the square format, you want to be able to project transparencies, or you wish to use a waist-level finder and avoid the inconvenience of rotating the camera when shooting vertical compositions. Otherwise, they are pretty much the same thing, IMO.
Larger formats: Have gotten very satisfying results from a Fuji GSW690 III - when I held the camera really steady! These big cameras were once a ubiquitous sight in Japan, where they were used to photograph groups of tourists at scenic locations. But the cameras were generally mounted atop a tripod. But for mixed shooting styles, where there might be some camera shake, I’d prefer the more economical 6x4.5 format